Method for producing various semiconductor optical devices of differing optical characteristics

ABSTRACT

An optical functional device includes a semiconductor substrate, an optical functional layer provided on said semiconductor substrate and selected from the group consisting of a light emitting layer, a light absorbing layer, and an optical waveguide layer. The optical functional layer has a multi-quantum well layer. Preferably, the semiconductor substrate is a nonplanar semiconductor substrate having a ridge and two grooves adjacent said ridge, said ridge having a ridge width of from 1 to 10 μm, a ridge height of from 1 μm to 5 μm, and a gap distance of from 1 μm to 10 μm. Such an optical functional device can be fabricated by growing, on a nonplanar semiconductor substrate having a specified dimension of the ridge, a strained multi-quantum well layer by metalorganic vapor phase epitaxy. Integrated optical device or circuit preferably includes an optical functional device on the nonplanar semiconductor substrate of a specified range of ridge shape factors. An integrated optical device can be fabricated by combination of a plurality of optical functional devices having slightly different compositions and including a part of a strained multi-quantum well layer monolithically grown on a nonplanar semiconductor substrate.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to semiconductor optical functionaldevices which can be used in the field of optical communication and moreparticularly to a plurality of optical functional devices which havelight emitting or receiving wavelengths that differ slightly one fromanother. The present invention also relates to methods for producingsuch devices. Optical functional devices include light emitting devices,light detecting devices, optical waveguides, spot-size converters,wavelength converters, optical couplers, optical splitters, etc. Makingthe most of their wide band and high directivity, semiconductor lightemitting devices can be used widely and are applicable to light sourcesfor optical measurement, integrated light sources, wavelength-tunablelight sources, light sources for optical communication, etc. Forexample, semiconductor light emitting devices are used, in the field ofoptical communication, as a multi-wavelength integrated light sourcewhich allows high density communication in the order of terabit, whilein the field of optical measurement, they are useful as light sourcesfor fiber gyros or OTDR (optical time-domain reflectometry).

Further, the present invention relates to semiconductor integratedoptical devices which can be used in optical processing such as opticalinformation processing and optical switching and more particularly to anoptical integrated device which includes a semiconductor substratehaving provided thereon at least one semiconductor device having opticaltransmitting and receiving functions. The present invention also relatesto a method for producing such semiconductor integrated optical devices.

DESCRIPTION OF RELATED ART

In coping with increasing sophistication of optical communicationsystems or methods, there has been progressing investigation on anoptical integrated circuit which includes a semiconductor substratehaving arranged thereon a plurality of optical functional devices suchas a light emitting device, an optical waveguide, and a light detectingdevice and which enables processing of many optical communicationsignals at a time en bloc. In order to provide an effective opticalintegrated circuit, it is of urgent necessity to develop opticalfunctional devices that can process a plurality of optical beams havingcommunication wavelengths which differ slightly one from another.Therefore, there is a keen desire to develop an optical functionaldevice which includes a semiconductor substrate having provided thereona plurality of light emitting devices having oscillating wavelengthsslightly different from each other.

As for the method for varying oscillating wavelengths of light emittingdevices, there has been generally used, for example, a method which usesa DFB (distributed feedback) laser diode or a DBR (distributed Braggreflector) laser diode whose semiconductor multilayer has incorporatedtherein a plurality of diffraction gratings of various frequenciescorresponding to their oscillating wavelengths, respectively. However,this method is disadvantageous in that the variation of wavelength islimited at most to a range of about 100 nm since the range of wavelengthin which a single light emitting device can oscillate depends on theoscillating region of the active layer, i.e., gain width, and that theoscillating characteristics of the light emitting device variesdepending on the wavelength range.

Also, there has been used a method in which the oscillating wavelengthof a semiconductor is varied depending on the temperature by making useof temperature dependence of its bandgap. This method, however, isdisadvantageous in that the response of the device is relatively slow,the device is not suited for integration, and the oscillatingcharacteristics of the device other than oscillating wavelength alsovary concomitantly.

On the other hand, in order to slightly vary the oscillating wavelengthcharacteristics or wavelength sensitivity characteristics of lightemitting devices or detectors, respectively, arranged on a semiconductorsubstrate two-dimensionally, it is usually necessary to fabricatesemiconductor layers having crystal compositions corresponding torespective wavelengths. According to conventional semiconductor growthtechnologies, a plurality of semiconductor layers having crystalcompositions slightly differing from each other have been fabricated onthe same semiconductor substrate by repeating growth of a crystalseveral times.

Several methods have been proposed or tried which grow crystals havingdifferent light emitting characteristics on the same substrate. However,none of them is a decisive technology which is superior to the methodthat involves repeating growth several times.

For example, as the method in which in order to vary wavelengths of aplurality of optical functional devices, the widths of the respectiveactive layers are made different slightly from each other, there can becited, for example, a method in which there are arranged on asemiconductor substrate stripe masks made of an oxide or nitride filmhaving predetermined widths corresponding to those of active layers oroptical waveguides to be fabricated, and crystals are grown between thestripes (usually referred to as "mask selection growth method", cf.,e.g., Aoki, et al., Oyo Denshi Bussei Bunkakai Kenkyu Hokoku No. 445,p9-14 (Applied Electronic Physical Property Meeting Study Report No.445, p9-14).

FIGS. 1A to 1C illustrate the above-mentioned masked selective growthmethod. FIG. 1A is a perspective view showing a planar semiconductorsubstrate 1 on which there are formed, for example, two oxide or nitridefilms 2a in the form of stripes having predetermined dimensions. Wm is awidth of each stripe, and Wg is a width of the gap between the twostripes 2a. On the substrate shown in FIG. 1A, there is formed asemiconductor multilayer by crystal growth. FIG. 1B is a perspectiveview showing the semiconductor substrate of FIG. 1A after the selectivecrystal growth. As a result of the selective growth, the substrate 1 isprovided with a semiconductor multilayer grown on regions other than themasks 2a to form a ridge 3 with a groove or gap 4 on each side thereof.The semiconductor multilayer includes a buffer layer, an active layer oroptical waveguide layer 6, a cladding layer 7 and a contact layer 10.FIG. 1C is a graph illustrating an example of characteristics, orrelationship between mask width (Wm) and oscillating wavelength(photoluminescence (PL) wavelength), at various gap distances (Wg), ofIn_(1-x) Ga_(x) As/InGaAsP crystal (λg=1.15 μm) fabricated by thecrystal growth.

The amount of shift of oscillating wavelength increases with an increasein the mask width, Wm, and with a decrease in the mask distance, Wg.This is because the source materials migrate from on the masks into thegap to vary the growth rate. As a result, there occur changes in thethicknesses of various semiconductor layers on the mesa structure,particularly quantum well layers, and the changes give rise to changesin the light emitting characteristics of the resulting semiconductordevice.

However, the above-described crystal growth method is inevitablysusceptible to influences by the mask such as incorporation ofimpurities from the mask. Hence, the product of the above method hascrystal quality which is not always superior to that of thesemiconductor device fabricated without masks. In addition, during theprogress of process steps, the masks must be removed after the growth ofcrystals. This makes the fabrication process complicated.

To obtain a shift of the oscillating wavelength of a light emittingdevice in the order of about 100 nm, masks to be used must extend about100 μm in a transverse direction, which makes it difficult to use theabove-described selective growth method in the integration of such lightemitting devices in a minute region as wide as several μm. When thewavelength is varied largely, the position of the light emitting layeris shifted longitudinally in a direction of the cavity, resulting indeteriorated light coupling.

On the other hand, variation of the oscillating characteristics of alight emitting device by slightly changing the crystal composition ofthe active layer without using masks has been achieved conventionallyby, for example, irradiation of laser beam during the crystal growthutilizing an metalorganic molecular beam epitaxy (MOMBE method) asdescribed in Yamada, et al., Oyo Denshi Bussei Bunkakai Kenkyu HokokuNo. 445, p27-32 (Applied Electronic Physical Property Meeting StudyReport No. 445, p27-32). However, this method is disadvantageous in thatit must use a special, expensive apparatus, and that it fails tofabricate a plurality of crystals having slightly different compositionsin a relatively wide region of the semiconductor device.

There is known a light emitting device which includes a ridged ornonplanar semiconductor substrate on which semiconductor layers aregrown. Japanese Patent Application Laying-open No. 4-305991 (1992)describes realization of a plurality of light emitting devices havingslightly different oscillating wavelengths by growing, by molecular beamepitaxy (MBE), strained quantum well active layers on a nonplanarsemiconductor substrate having a plurality of ridges which differslightly in width from each other.

However, this method provides a light emitting device which utilizesexciton transition energy of strained quantum wells. When the ridgewidth of the nonplanar substrate is set to 2 μm, 3 μm, 5 μm, or 8 μm,the oscillating wavelength of the device is varied to 1.52 μm, 1.53 μm,1.54 μm, or 1.55 μm, respectively. However, the amount of variation israther small so that the amount of shift of wavelength is not so highlycontrollable with the change in ridge width alone.

Japanese Patent Application Laying-open No. 4-364084 (1992) discloses amethod for fabricating a multiple wavelength laser diode by growingsemiconductor layers on a nonplanar semiconductor substrate havingridges of varied widths and distances (gap distances) to vary thethicknesses of the active layers in the ridges, respectively, therebyvarying their oscillating wavelengths. On the other hand, JapanesePatent Application Laying-open No. 4-206982 (1992) describes a lightemitting device fabricated by growing semiconductor layers bymetalorganic vapor phase epitaxy (MOVPE) on a semiconductor substratehaving ridges each of which is sandwiched by two types of grooves ofdifferent widths. The light emitting device is a so-calledwindow-structure laser diode at each terminal end of which there isformed a semiconductor layer having a large bandgap so that thedeterioration of terminal end surfaces can be prevented and higheroutput can be obtained.

Further, there have been known functional superluminescent diodes whichare produced by growing semiconductor layers having differentcompositions along its length in order to get wide-band light emittingcharacteristics (Noguchi, et al., Denshi Joho Tsushin Gakkai OQE 91-83(1991) (The Institute of Electronics, Information and CommunicationEngineers OQE 91-83 (1991)). The functional superluminescent diode isfabricated by performing crystal growth twice for forming light emittingportions having different compositions. Hence, it is troublesome toperform crystal growths and a special countermeasure is required to makeas small as possible a waste growth area existing between the both lightemitting portions.

Although it could in principle have been possible to fabricate anemitter having three or more different light emitting regions, a hightechnology in the aspects of crystal growth and processing process isrequired to fabricate such a light emitting device. In fact, there havebeen known few light emitting devices that have three or moresemiconductor devices having different compositions integrated in alongitudinal direction.

Conventionally, most of transceivers with terminal optical amplifiersused in optical communication are combinations of individual componentssuch as splitters, LD, PD, etc. There are also those which includeoptical guides made of quartz or the like material, and laser diodes andphotodetectors made of semiconductors, respectively, are connected toeach other with solder, for example, or coupled via optical fibers orthe like. Further, there are those which have necessary semiconductorparts or components integrated on one semiconductor substrate.

However, the transceivers constructed with the semiconductor componentsand/or quartz guides as described above have various disadvantages. Theytend to have larger sizes since there is needed space for connectingvarious components or for other reasons, and as a result adjustmentbecomes troublesome, and a complicated fabrication process becomesnecessary, which is not suited for mass production. In addition, theircoupling coefficient is insufficient. In the case of integratedsemiconductor circuits, semiconductors having various bandgaps must befabricated on one semiconductor substrate and, hence, a very complicatedfabrication process is necessary which involves removal of a part of aonce grown crystal and growing again a semiconductor crystal which has acomposition different from that of the previously grown crystal, orthere is needed high techniques. At present, this monolithic integrationtechniques cannot be a candidate of techniques which substitute for theabove-described hybridizing connection of individual components.

On the other hand, for obtaining more improved optical communicationsystems and optical information processing systems in view ofperformance, functionality, and costs, it is an essential technique tointegrate semiconductor optical devices. In the integration ofsemiconductor optical devices, individual semiconductor optical devicesare connected to each other through glass optical circuits or opticalfibers. Even when a plurality of devices are integrated monolithicallyon one and the same semiconductor substrate, optical signals areinputted in and outputted from the integrated device through glassoptical circuits or optical fibers.

Integration of optical devices, unlike integration of electronicdevices, is vulnerable to connection losses upon transmission andreceipt of light between the devices. This is mainly because the opticalspot size on the side of a semiconductor device is smaller than theoptical spot size on the side of a glass optical circuit or an opticalfiber. In order to avoid this problem and alleviate allowance forconnection, there has been proposed a device which converts relativelysmall spot size on the side of a semiconductor device to a relativelylarge spot size on the side of a glass optical circuit or an opticalfiber.

In the case where an optical waveguide is used to convert the spot sizeof an optical device, the core of the optical waveguide must have astructure which varies in a direction of light transmission in at leastone physical property selected from thickness, width, and difference inrefractive index with respect to the cladding. Usually, semiconductoroptical waveguides are fabricated by processing epitaxially grown layersby dry etching or the like technique and performing epitaxial regrowth.Therefore, conversion of the spot size of optical devices using opticalwaveguides suffers from various problems on production such ascomplicated process steps, deterioration of interfaces during theprocessing, and contamination of impurities.

FIG. 2 is a schematic perspective view showing the spot size converterdescribed in ELECTRONICS LETTERS 26th Mar. 1992 Vol. 28, No. 7, p-631 asan example of the current technique. This spot size converter iscomposed of three portions, a first portion I, a second portion II, anda third portion III. The first portion I includes an optical waveguidehaving a smaller spot and for optical connection to the chip itself. Thesecond portion II has two tapered layers which convert the spot size. Anupper optical waveguide 6u has a width which is decreased linearlytoward an end to form a tip end while a lower optical waveguide 6_(L)has a width which is increased linearly toward an end to the same widthas an output optical waveguide in the third portion III.

The spot size converter includes a semi-insulating InP wafer 1a as asubstrate, an InP buffer layer 5 grown on the substrate 1a bymetalorganic vapor phase epitaxial growth (MOVPE), and two layersepitaxially grown on the buffer layer 5 to provide the upper and lowertapered optical waveguides 6_(U) and 6_(L). The upper and lower layers6_(U) and 6_(L) are made of InGaAsP's having different bandgaps. Theselayers are each tapered by dry etching in two steps. On the layers 6_(U)and 6_(L) there is formed an InP layer 7a by MOVPE growth.

As described above, the fabrication of this spot size converter requiresat least two epitaxial growth steps and at least two etching steps and,hence, its fabrication process is complicated.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an optical functionaldevice, particularly a semiconductor light emitting device which isintegrated on a semiconductor substrate having an unevenness pattern(ridges) by crystal growth and which has a wide-band oscillatingcharacteristics.

Another object of the present invention is to provide a monolithicallyintegrated optical device having a plurality of such optical functionaldevices on a semiconductor substrate.

Still another object of the present invention is to provide a method forfabricating such an optical functional device.

Yet another object of the present invention is to provide a method forfabricating such a monolithically integrated optical device.

Further object of the present invention is to provide a method forselecting an oscillating wavelength of a semiconductor light emittingdevice.

According to the present invention, there is provided an opticalfunctional device comprising: a semiconductor substrate; an opticalfunctional layer provided on the semiconductor substrate and selectedfrom the group consisting of a light emitting layer, a light absorbinglayer, and an optical waveguide layer; the optical functional layercomprising a multi-quantum well layer; and the semiconductor substratebeing a nonplanar semiconductor substrate having a ridge and two groovesadjacent the ridge, the ridge having a ridge width of from 1 to 10 μm, aridge height of from 1 μm to 5 μm, and a gap distance of from 1 μm to 10μm.

Here, the ridge may have an upper face and a side face; and the ridgemay have a diffractive grating on at least one of the upper and sidesurfaces.

Also, there is provided an optical functional device comprising: anonplanar semiconductor substrate; a ridge provided on the nonplanarsemiconductor substrate, the ridge having an upper face and a side face;an optical functional layer provided on the nonplanar semiconductorsubstrate and selected from the group consisting of a light emittinglayer, a light absorbing layer, and an optical waveguide layer; anactive layer formed in the optical functional layer and having amulti-quantum well structure, the active layer having a first portionand a second portion, the first portion being provided above the ridgeand the second portion being provided above a portion of thesemiconductor substrate other than the ridge; the first portion of theactive layer having a composition different from a composition of thesecond portion of the active layer; and a diffractive grating providedon at least one of the upper face and the side face of the ridge.

Here, the ridge width may be from 1 μm to 5 μm; and the multi-quantumwell structure may comprise a monolithic crystal having differentcompositions in a direction of a cavity; the optical functional devicehaving oscillating or detecting characteristics varying in a directionof the cavity.

The upper face of the ridge may have a diffractive grating whose pitchis varied in accordance with the composition of the crystal above theridge; the optical functional device having a facet through which lighttransmits, and emitting a plurality of light beams at singlelongitudinal mode out of the facet or receiving a light beam or beamshaving a wavelength or wavelengths shorter than a predeterminedwavelength through the facet.

The ridge may have provided therein an optical guide layer.

The ridge width may be from 1 μm to 5 μm; the ridge having a shape whoseridge width, ridge height or gap distance being varied in a transversedirection; the optical functional device having oscillating or detectingcharacteristics which vary in the transverse direction.

The upper face of the ridge may have a diffractive grating whose pitchis varied in accordance with the composition of the crystal above theridge; the optical functional device emitting a light beam at singlelongitudinal mode or receiving a light beam or beams having a wavelengthor wavelengths shorter than a predetermined wavelength in a directionparallel to the direction of cavity.

The optical functional layer may be a semiconductor light emittinglayer; and the grooves may be formed thereon with a semiconductor thinfilm layer having a conductivity type different from that of thesemiconductor substrate.

The optical functional layer may be a semiconductor light emittinglayer; and wherein the ridge on the nonplanar semiconductor substratehas a ridge shape of anti-mesa structure.

The optical functional layer may be a semiconductor light emittinglayer; and the nonplanar semiconductor substrate may have formed thereona semiconductor buffer layer.

Further, there is provided an integrated optical device comprising: anonplanar semiconductor substrate having a ridge shape; a plurality ofoptical functional devices arranged on the nonplanar semiconductorsubstrate and selected from the group consisting of a light emittingdevice and a detecting device; the plurality of optical functionaldevices being connected to each other operationally so thatcharacteristics of the plurality of optical functional devices can becombined functionally; the plurality of optical functional devices eachcomprising a part of a strained multi-quantum well layer formed on themonolithically; at least a part of the plurality of optical functionaldevices having a composition or respective compositions different incomposition from each other.

The nonplanar semiconductor substrate may be provided with a ridgehaving a ridge width of from 1 to 10 μm, a ridge height of from 1 μm to5 μm, and a gap distance of from 1 μm to 10 μm.

The ridge may have an upper face and a side face; and the ridge may havea diffractive grating on at least one of the upper and side surfaces.

The diffractive grating may have a period varying in accordance with aposition at which the diffractive grating is arranged.

Still further, there is provided an integrated optical devicecomprising: a nonplanar semiconductor substrate; a ridge provided on thenonplanar semiconductor substrate, the ridge having an upper face and aside face; a light emitting portion provided on the nonplanarsemiconductor substrate and including an optical functional device; adetecting portion provided on the nonplanar semiconductor substrate andincluding an optical device; the light emitting portion and thedetecting portion being arranged in parallel to and optically connectedeach other; a semiconductor optical waveguide optically connected to thelight emitting portion and the detecting portion, the optical waveguideincluding an optical functional device and having a common inputting andoutputting portion connected to the optical functional device; theoptical functional devices being connected to each other operationallyso that characteristics of the optical functional devices can becombined functionally; the optical functional device in the lightemitting portion having a light emitting device which emits light havinga first wavelength and a detecting device which detects an output of thelight emitting device; the optical functional device in the detectingportion having a detecting device which detects the light having thefirst wavelength, a wavelength filter which absorbs the light having thefirst wavelength, and a detecting device which detects light having asecond wavelength; and the integrated optical device having a functionof emitting and detecting two light beams which have differentwavelengths and which propagate in the common inputting and outputtingportion.

Also, there is provided an integrated optical device comprising: anonplanar semiconductor substrate; a ridge provided on the nonplanarsemiconductor substrate; a light emitting portion which is provided onthe nonplanar semiconductor substrate which exhibits a light emittingfunction and a detecting function; a detecting portion which is providedon the nonplanar semiconductor substrate and which exhibits a detectingfunction; a semiconductor optical waveguide which is optically connectedto the light emitting portion and the detecting portion; the lightemitting portion having a light emitting device which emits light havinga first wavelength and a reflector having a diffractive grating, thelight emitting portion exhibiting a light emitting function and afunction of detecting the light having the first wavelength; thedetecting portion having a wavelength filter which absorbs light havingthe first wavelength, and a detecting device which detects light havinga second wavelength, the detecting portion exhibiting a function ofdetecting a wavelength filter which absorbs the light haven; theintegrated optical device exhibiting a function of emitting anddetecting two light beams which have wavelengths differing from eachother and which propagate in the common inputting and outputtingportion.

Further, there is provided an optical device comprising: a nonplanarsemiconductor substrate; a ridge provided on the nonplanar semiconductorsubstrate; an optical waveguide provided on the nonplanar semiconductorsubstrate; a quantum well layer formed on the optical waveguide andhaving a multi-quantum well structure; the ridge having a dimensionvaried from one end thereof toward another end thereof so that thequantum well layer has a composition or thickness which is varied formone end toward another end of the optical waveguide and a light wavepropagating in the waveguide is converted of its spot size.

Still further, there is provided a method for fabricating an opticalfunctional device having at least one optical functional layer selectedfrom the group consisting of a light emitting layer, an absorbing layerand an optical waveguide layer, comprising the steps of: providing anonplanar semiconductor substrate having a ridge of which a ridge widthis from 1 μm to 10 μm, a ridge height is from 1 μm to 5 μm, and a gapdistance is from 1 μm to 10 μm; and growing a strained multi-quantumwell layer on the nonplanar semiconductor substrate by metalorganicvapor phase epitaxy.

Here, the method may further comprise the steps of: providing a planarsemiconductor substrate; forming a semiconductor protective thin filmlayer having a composition different from that of the planarsemiconductor substrate; and processing the planar substrate having theprotective thin film layer to render the planar substrate nonplanar.

Yet further, there is provided a method for controlling opticalcharacteristics of an optical functional device having a semiconductorsubstrate, at least one optical functional layer selected from the groupconsisting of a light emitting layer, a detecting layer and an opticalwaveguide layer, the method comprising the steps of: providing as thesemiconductor substrate a nonplanar semiconductor substrate having aridge of a selected shape of which a ridge width is from 1 μm to 10 μm,a ridge height is from 1 μm to 5 μm, and a gap distance is from 1 μm to10 μm; and growing a multi-quantum well structure on the nonplanarsemiconductor substrate by metalorganic vapor phase epitaxy to form theat least optical functional layer; whereby varying a composition of themulti-quantum well structure formed on the ridge to change opticalcharacteristics hereof.

Here, the optical characteristics may be a bandgap or refractive indexof the quantum well structure, and at least one of the ridge width,ridge height and gap distance may be varied from one end toward anotherend of the waveguide on the ridge.

The optical characteristics may be a light emitting characteristics ordetecting characteristics of the optical functional device; and at leastone of the ridge width, ridge height and gap distance may be varied in alongitudinal direction, so that the light emitting characteristics orthe detecting characteristics of the optical functional device can bevaried in a direction of a cavity.

The optical characteristics may be a light emitting characteristics ordetecting characteristics of the optical functional device; and at leastone of the ridge width, ridge height and gap distance may be varied in atransverse direction to form an array-like optical functional device, sothat the light emitting characteristics or the detecting characteristicsof the optical functional device can be varied in a transversedirection.

The above and other objects, effects, features and advantages of thepresent invention will become more apparent from the followingdescription of the embodiment thereof taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective view showing a semiconductor substrate during aprocess for fabricating a strained quantum well structure by a maskedselective growth method;

FIG. 1B is a perspective view showing a semiconductor substrate during aprocess for fabricating a strained quantum well structure by a maskedselective growth method;

FIG. 1C is a graph illustrating the dependence of oscillating wavelengthon mask width at various gap distances;

FIG. 2 is a schematic perspective view showing a conventional spot sizeconverter;

FIG. 3 is a graph illustrating the dependence of oscillating wavelengthon composition of a strained quantum well structure at variousthicknesses;

FIG. 4A is a cross sectional view showing an example of a structure of a1.55 μm band light emitting device formed on a nonplanar semiconductorsubstrate;

FIG. 4B is a graph illustrating results of simulation of oscillation inthe structure shown in FIG. 4A, with solid and broken lines indicatingdifferences in crystal properties;

FIG. 5A is a perspective view showing a semiconductor substrate during aprocess for fabricating a nonplanar substrate, the substrate beingformed thereon with a mask for etching;

FIG. 5B is a perspective view showing a semiconductor substrate during afabrication process, the substrate being formed thereon with a mask forselective growth;

FIG. 5C is a perspective view showing a semiconductor substrate during afabrication process, the substrate being formed therein with grooves;

FIG. 6A is a graph illustrating a shift in the bandgap wavelength, asinfluenced by ridge width, of a multi-quantum well structure formed on aridge;

FIG. 6B is a graph illustrating a shift in the bandgap wavelength, asinfluenced by ridge height, of a multi-quantum well structure formed ona ridge;

FIG. 7 is a graph illustrating dependence, on ridge and gap distance, ofa photoluminescence peak wavelength of a quantum well structure grown ona nonplanar semiconductor substrate;

FIG. 8 is a graph illustrating dependence, on ridge height, of aphotoluminescence peak wavelength of a quantum well structure grown on anonplanar semiconductor substrate;

FIG. 9A is a perspective view showing a semiconductor substrate during aprocess for fabricating an optical functional device according to afirst embodiment of the present invention, the semiconductor substratebeing formed thereon with a mask for etching;

FIG. 9B is a perspective view showing a semiconductor substrate during aprocess for fabricating an optical functional device according to afirst embodiment of the present invention, the semiconductor substratebeing formed thereon with a mask for selective growth;

FIG. 9C is a perspective view showing a semiconductor substrate during aprocess for fabricating an optical functional device according to afirst embodiment of the present invention, the semiconductor substratebeing formed therein with grooves to form a nonplanar substrate;

FIG. 9D is a perspective view showing a semiconductor substrate during aprocess for fabricating an optical functional device according to afirst embodiment of the present invention, the semiconductor substratebeing formed thereon with an active layer by a crystal growth (growth ofan active layer);

FIG. 9E is a perspective view showing a semiconductor substrate during aprocess for fabricating an optical functional device according to afirst embodiment of the present invention, the semiconductor substratebeing buried by a crystal growth (burying growth);

FIG. 9F is a perspective view showing a semiconductor substrate during aprocess for fabricating an optical functional device according to afirst embodiment of the present invention, the semiconductor substratebeing formed thereon with an electrodes;

FIG. 10A ms a graph illustrating dependence, on ridge width (dw), ofoscillating wavelength of an optical functional device according to afirst embodiment of the present invention;

FIG. 10B ms a graph illustrating dependence, on gap distance (dg), ofoscillating wavelength of an optical functional device according to afirst embodiment of the present invention;

FIG. 10C is a graph illustrating dependence, on ridge height, h, ofoscillating wavelength of an optical functional device according to afirst embodiment of the present invention;

FIG. 11A is a graph illustrating dependence, on ridge width, ofthickness of InGaAs quantum well layer of an optical functional deviceaccording to a first embodiment of the present invention;

FIG. 11B is a graph illustrating dependence, on gap distance, ofthickness of InGaAs quantum well layer of an optical functional deviceaccording to a first embodiment of the present invention;

FIG. 11C is a graph illustrating dependence, on ridge height, ofthickness of InGaAs quantum well layer of an optical functional deviceaccording to a first embodiment of the present invention;

FIG. 12A is a perspective view showing a nonplanar semiconductorsubstrate according to a second embodiment of the present invention,with diffractive gratings being formed on a ridge;

FIG. 12B is a perspective view showing a nonplanar semiconductorsubstrate according to a second embodiment of the present invention,with diffractive gratings being formed in an upper surface of an opticalwaveguide layer provided on a ridge;

FIG. 12C is a perspective view showing a nonplanar semiconductorsubstrate according to a second embodiment of the present invention,with diffractive gratings being formed on a ridge;

FIG. 13A is a perspective view showing a semiconductor substrate duringa fabrication process (gap distance-modulated method) for fabricating afour-channel integrated laser diode array according to a thirdembodiment of the present invention, the semiconductor substrate beingformed thereon with a mask for etching;

FIG. 13B is a perspective view showing a semiconductor substrate duringa fabrication process (gap distance-modulated method) for fabricating afour-channel integrated laser diode array according to a thirdembodiment of the present invention, the semiconductor substrate beingformed therein with grooves to form a nonplanar semiconductor substrate;

FIG. 13C is a perspective view showing a semiconductor substrate duringa fabrication process (gap distance modulated method) for fabricating afour-channel integrated laser diode array according to a thirdembodiment of the present invention, the semiconductor substrate beingformed thereon with an active layer by crystal growth (growth of anactive layer);

FIG. 13D is a perspective view showing a semiconductor substrate duringa fabrication process (gap distance modulated method) for fabricating afour-wavelength integrated laser diode array according to a thirdembodiment of the present invention, the semiconductor substrate beingburied by crystal growth (burying growth);

FIG. 13E is a perspective view showing a semiconductor substrate duringa fabrication process (gap distance modulated method) for fabricating afour-wavelength integrated laser diode array according to a thirdembodiment of the present invention, the semiconductor substrate beingformed thereon with electrodes;

FIG. 14 is a graph illustrating oscillating characteristics of afour-channel laser diode array according to a third embodiment of thepresent invention;

FIG. 15A is a perspective view showing a semiconductor substrate duringa process (gap distance-modulated method) for fabricating a four-channelintegrated DFB laser diode array according to a fourth embodiment of thepresent invention, the semiconductor substrate being formed thereon withdiffractive gratings;

FIG. 15B is a perspective view showing a semiconductor substrate duringa process for fabricating a four-channel integrated DFB laser diodearray according to a fourth embodiment of the present invention, thesemiconductor substrate being formed thereon with a mask for etching;

FIG. 15C is a perspective view showing a semiconductor substrate duringa fabrication process (gap distance-modulated method) of a four-channelintegrated DFB laser diode array according to a fourth embodiment of thepresent invention, the semiconductor substrate being formed therein withgrooves to form a nonplanar substrate;

FIG. 15D is a perspective view showing a semiconductor substrate duringa process for fabricating a four-channel integrated DFB laser diodearray according to a fourth embodiment of the present invention, thesemiconductor substrate being formed thereon with an active layer by acrystal growth (growth of an active layer);

FIG. 15E is a perspective view showing a semiconductor substrate duringa process for fabricating a four-channel integrated DFB laser diodearray according to a fourth embodiment of the present invention, thesemiconductor substrate being buried by a crystal growth (buryinggrowth);

FIG. 15F is a perspective view showing a semiconductor substrate duringa process for fabricating a four-channel integrated DFB laser diodearray according to a first embodiment of the present invention, thesemiconductor substrate being formed thereon with electrodes;

FIG. 16 is a graph illustrating oscillating characteristics of afour-channel laser diode array according to a fourth embodiment of thepresent invention;

FIG. 17A is a perspective view showing a semiconductor substrate duringa process for fabricating a four-channel integrated laser diode arrayaccording to a fifth embodiment of the present invention, thesemiconductor substrate being formed thereon with a mask for etching;

FIG. 17B is a perspective view showing a semiconductor substrate duringa fabrication process (ridge width-modulated method) of a four-channelintegrated laser diode array according to a fifth embodiment of thepresent invention, the semiconductor substrate being formed therein withgrooves to form a nonplanar substrate;

FIG. 17C is a perspective view showing a semiconductor substrate duringa process for fabricating four-channel integrated laser diode arrayaccording to a fifth embodiment of the present invention, thesemiconductor substrate being formed thereon with an active layer by acrystal growth (growth of an active layer);

FIG. 17D is a perspective view showing a semiconductor substrate duringa process for fabricating a four-channel integrated laser diode arrayaccording to a fifth embodiment of the present invention, thesemiconductor substrate being buried by a crystal growth (buryinggrowth);

FIG. 17E is a perspective view showing a semiconductor substrate duringa process for fabricating a four-channel integrated laser diode arrayaccording to a fifth embodiment of the present invention, thesemiconductor substrate being formed thereon with electrodes;

FIG. 18 is a schematic perspective view showing a monolithic heterodynereceiver according to a seventh embodiment of the present invention;

FIG. 19A is a perspective view showing a semiconductor substrate duringa fabrication process (gap distance-modulated method) of asuperluminescent diode according to an eighth embodiment of the presentinvention, the semiconductor substrate being formed thereon with a maskfor etching;

FIG. 19B is a perspective view showing a semiconductor substrate duringa fabrication process (gap distance-modulated method) of asuperluminescent diode according to an eighth embodiment of the presentinvention, the semiconductor substrate being formed therein with groovesto form a nonplanar substrate;

FIG. 19C is a perspective view showing a semiconductor substrate duringa process for fabricating a superluminescent diode according to aneighth embodiment of the present invention, the semiconductor substratebeing formed thereon with an active layer by a crystal growth (growth ofan active layer);

FIG. 19D is a perspective view showing a semiconductor substrate duringa process for fabricating a superluminescent diode according to aneighth embodiment of the present invention, the semiconductor substratebeing buried by a crystal growth (burying growth);

FIG. 19E is a perspective view showing a semiconductor substrate duringa process for fabricating a superluminescent diode according to aneighth embodiment of the present invention, the semiconductor substratebeing formed thereon with electrodes;

FIG. 20 is a graph illustrating oscillating spectrum characteristics ofa superluminescent diode according to an eighth embodiment of thepresent invention;

FIG. 21A is a perspective view showing a semiconductor substrate duringa fabrication process (ridge width-modulated method) of asuperluminescent diode according to a ninth embodiment of the presentinvention, the semiconductor substrate being formed thereon with a maskfor etching;

FIG. 21B is a perspective view showing a semiconductor substrate duringa fabrication process (ridge width-modulated method) of asuperluminescent diode according to a ninth embodiment of the presentinvention, the semiconductor substrate being formed therein with groovesto form a nonplanar substrate;

FIG. 21C is a perspective view showing a semiconductor substrate duringa process for fabricating a superluminescent diode according to a ninthembodiment of the present invention, the semiconductor substrate beingformed thereon with an active layer by a crystal growth (growth of anactive layer);

FIG. 21D is a perspective view showing a semiconductor substrate duringa process for fabricating a superluminescent diode according to a ninthembodiment of the present invention, the semiconductor substrate beingformed thereon a mask for etching;

FIG. 21E is a perspective view showing a semiconductor substrate duringa process for fabricating a superluminescent diode according to a ninthembodiment of the present invention, the semiconductor substrate beingdry-etched to form a nonplanar substrate for burying growth;

FIG. 22A is a perspective view showing a semiconductor substrate duringa fabrication process (ridge height-modulated method 1) of asuperluminescent diode according to a tenth embodiment of the presentinvention, the semiconductor substrate being formed thereon with a maskfor selective growth;

FIG. 22B is a perspective view showing a semiconductor substrate duringa fabrication process (ridge height-modulated method 1) of asuperluminescent diode according to a tenth embodiment of the presentinvention, the semiconductor substrate having become a nonplanarsubstrate (after removal of the mask);

FIG. 22C is a perspective view showing a semiconductor substrate duringa process for fabricating a superluminescent diode according to a tenthembodiment of the present invention, the semiconductor substrate beingformed thereon a mask for etching;

FIG. 22D is a perspective view showing a semiconductor substrate duringa process for fabricating a superluminescent diode according to a tenthembodiment of the present invention, the semiconductor substrate being anonplanar substrate for crystal growth with its ridge height beingmodulated;

FIG. 22E is a perspective view showing a semiconductor substrate duringa process for fabricating a superluminescent diode according to a tenthembodiment of the present invention, the semiconductor substrate beingformed thereon with an active layer by a crystal growth (growth of anactive layer);

FIG. 23A is a perspective view showing a semiconductor substrate duringa fabrication process (ridge height-modulated method 2) of asuperluminescent diode according to a variation of the tenth embodimentof the present invention, the semiconductor substrate being formedthereon with a mask for selective growth;

FIG. 23B is a perspective view showing a semiconductor substrate duringa fabrication process (ridge height-modulated method 2) of asuperluminescent diode according to a variation of the tenth embodimentof the present invention, the semiconductor substrate having become anonplanar substrate (after removal of the mask);

FIG. 23C is a perspective view showing a semiconductor substrate duringa process for fabricating a superluminescent diode according to avariation of the eleventh embodiment of the present invention, thesemiconductor substrate being formed thereon a mask for etching;

FIG. 24A is a perspective view showing a semiconductor substrate duringa process for fabricating an optical device according to an eleventhembodiment of the present invention, the semiconductor substrate beingformed thereon with an oxide mask for etching;

FIG. 24B is a perspective view showing a semiconductor substrate duringa process for fabricating an optical device according to an eleventhembodiment of the present invention, the semiconductor substrate beingformed therein with grooves to form a nonplanar substrate;

FIG. 24C is a perspective view showing a semiconductor substrate duringa process for fabricating an optical device according to an eleventhembodiment of the present invention, the nonplanar semiconductorsubstrate being formed thereon with a block layer;

FIG. 24D is a perspective view showing a semiconductor substrate duringa process for fabricating an optical device according to an eleventhembodiment of the present invention, the semiconductor substrate beingformed with a device structure by MOVPE growth;

FIG. 24E is a perspective view showing a semiconductor substrate duringa process for fabricating an optical device according to an eleventhembodiment of the present invention, the semiconductor substrate beingformed thereon with electrodes;

FIG. 25 is a graph illustrating the oscillating characteristics of anoptical device according to an eleventh embodiment of the presentinvention;

FIG. 26A is a perspective view showing a semiconductor substrate duringa process for fabricating an optical device according to a twelfthembodiment of the present invention, the semiconductor substrate beingformed thereon with an oxide mask for etching;

FIG. 26B is a perspective view showing a semiconductor substrate duringa process for fabricating an optical device according to a twelfthembodiment of the present invention, the semiconductor substrate beingformed therein with grooves to form a ridge of a mesa structure;

FIG. 26C is a perspective view showing a semiconductor substrate duringa process for fabricating an optical device according to a twelfthembodiment of the present invention, the semiconductor substrate beingformed therein with grooves to form a ridge of an anti-mesa structure;

FIG. 26D is a perspective view showing a semiconductor substrate duringa process for fabricating an optical device according to a twelfthembodiment of the present invention, the nonplanar semiconductorsubstrate being formed thereon with a device structure by MOVPE growth;

FIG. 26E is a perspective view showing a semiconductor substrate duringa process for fabricating an optical device according to a twelfthembodiment of the present invention, the semiconductor substrate beingformed thereon with an electrode;

FIG. 27 is a graph illustrating the optical characteristics of anoptical device according to a twelfth embodiment of the presentinvention;

FIG. 28A is a perspective view showing a semiconductor substrate duringa process for fabricating an optical device according to a thirteenthembodiment of the present invention, the semiconductor substrate beingformed thereon with an oxide mask for etching via a damage absorbinglayer;

FIG. 28B is a perspective view showing a semiconductor substrate duringa process for fabricating an optical device according to a thirteenthembodiment of the present invention, the semiconductor substrate beingformed therein with grooves to form a nonplanar substrate (prior toremoval of a damage absorbing layer);

FIG. 28C is a perspective view showing a semiconductor substrate duringa process for fabricating an optical device according to a thirteenthembodiment of the present invention, the nonplanar semiconductorsubstrate being formed thereon with a device structure by MOVPE growth;

FIG. 28D is a perspective view showing a semiconductor substrate duringa process for fabricating an optical device according to a thirteenthembodiment of the present invention, the semiconductor substrate beingformed thereon with electrodes;

FIG. 29 is a schematic plan view showing an integrated optical circuitaccording to a fourteenth embodiment of the present invention;

FIG. 30 is a schematic cross sectional view showing a semiconductorsubstrate during a process for fabricating an integrated optical circuitaccording to a fourteenth embodiment of the present invention, thesemiconductor substrate being formed thereon with a common opticalwaveguide and an n-InP layer;

FIG. 31 is a schematic plan view showing a semiconductor substrateduring a process for fabricating an integrated optical circuit accordingto a fourteenth embodiment of the present invention, the semiconductorsubstrate being formed in a portion of its n-InP layer with diffractivegratings;

FIG. 32 is a schematic plan view showing a semiconductor substrateduring a process for fabricating an integrated optical circuit accordingto a fourteenth embodiment of the present invention, the semiconductorsubstrate being formed therein with grooves to form ridges;

FIG. 33 is a schematic cross sectional view showing a semiconductorsubstrate during a process for fabricating an integrated optical circuitaccording to a fourteenth embodiment of the present invention, thesemiconductor substrate being formed thereon with a multi-quantum wellstructure;

FIG. 34 is a schematic cross sectional view showing a semiconductorsubstrate during a process for fabricating an integrated optical circuitaccording to a fourteenth embodiment of the present invention, thesemiconductor substrate being buried;

FIG. 35 is a schematic cross sectional view showing a semiconductorsubstrate during a process for fabricating an integrated optical circuitaccording to a fourteenth embodiment of the present invention, a dopedInP layer and a multi-quantum well structure having a 1.3 μm bandgapbeing etched off;

FIG. 36 is a schematic cross sectional view showing a semiconductorsubstrate during a process for fabricating an integrated optical circuitaccording to a fourteenth embodiment of the present invention, theetched-off portion being buried;

FIG. 37 is a graph illustrating dependence, on current, of opticaloutput of a 1.3 μm LD in an integrated optical device according to afourteenth embodiment of the present invention;

FIG. 38A is a graph illustrating dependence, on wavelength, ofphotocurrent of a PD in an integrated optical device according to afourteenth embodiment of the present invention;

FIG. 38B is a graph illustrating dependence, on wavelength, of crosstalkof a PD in an integrated optical device according to a fourteenthembodiment of the present invention;

FIG. 39A is a schematic cross sectional view showing a semiconductorsubstrate during a process for fabricating an integrated optical deviceaccording to a variation of a fourteenth embodiment of the presentinvention, the semiconductor substrate being formed therein two types ofgrooves with different gap distances;

FIG. 39B is a schematic cross sectional view showing a semiconductorsubstrate during a process for fabricating an integrated optical deviceaccording to a variation of a fourteenth embodiment of the presentinvention, the nonplanar semiconductor substrate being formed thereonwith a multi-quantum well structure;

FIG. 40 is a schematic plan view showing an integrated optical circuitaccording to a fifteenth embodiment of the present invention;

FIG. 41 is a schematic perspective view showing a semiconductorsubstrate for fabricating a spot size converter according to a sixteenthembodiment of the present invention, the semiconductor substrate beingformed with a ridge prior to epitaxial growth;

FIGS. 42A and 42B are respectively a schematic cross sectional viewshowing a front end of a spot size converter according to a sixteenthembodiment of the present invention, and a quantum well layer beingshown on an enlarged scale;

FIGS. 43A and 43B are respectively a schematic cross sectional viewshowing a rear end of a spot size converter according to a sixteenthembodiment of the present invention, with a quantum well layer beingshown on an enlarged scale;

FIG. 44 is a schematic perspective view showing a semiconductorsubstrate for fabricating a spot size converter according to aseventeenth embodiment of the present invention, the semiconductorsubstrate being formed with a ridge prior to epitaxial growth;

FIG. 45 is a schematic cross sectional view showing a front end of aspot size converter according to a seventeenth embodiment of the presentinvention, with a quantum well layer being shown on an enlarged scale;

FIG. 46 is a schematic cross sectional view showing a rear end of a spotsize converter according to a seventeenth embodiment of the presentinvention, with a quantum well layer being shown on an enlarged scale;

FIG. 47 is a schematic perspective view showing a semiconductorsubstrate for fabricating a spot size converter according to aneighteenth embodiment of the present invention, the semiconductorsubstrate being formed with a ridge prior to epitaxial growth;

FIG. 48 is a schematic cross sectional view showing a front end of aspot size converter according to an eighteenth embodiment of the presentinvention, with a quantum well layer being shown on an enlarged scale;and

FIG. 49 is a schematic cross sectional view showing a rear end of a spotsize converter according to an eighteenth embodiment of the presentinvention, with a quantum well layer being shown on an enlarged scale.

DESCRIPTION OF PREFERRED EMBODIMENTS

According to a basic aspect, the present invention relates to an opticalfunctional device such as an emitter, a detector or the like fabricatedbased on the phenomenon that the composition of a multi-quantum welllayer formed on a nonplanar semiconductor substrate varies depending onthe shape of its ridge. Also, the present invention relates to anoptical component or device having a portion with such aridge-shape-dependent composition. Further, the present inventionrelates to an integrated optical device including a plurality of suchoptical functional devices which are arranged on a semiconductorsubstrate and whose characteristics are combined functionally.

According to a first aspect of the present invention, a semiconductormultilayer composed of a strained multi-quantum well (MQW) structure asan active layer is formed by metalorganic vapor phase epitaxy (MOVPE) ona semiconductor substrate provided with at least one ridge whose shape,i.e., ridge width, distance between two adjacent ridges or between aridge and a side surface of a groove opposing the ridge (gap distance),and ridge height are set to dimensions within predetermined ranges,respectively.

When a semiconductor multilayer of a strained MQW structure is formed byan MOVPE method on a ridged or nonplanar semiconductor substrate ofwhich the ridge width, gap distance and ridge height are set topredetermined values in advance, the speed of migration of componentatoms which constitute the semiconductor differ on each crystallinesurfaces due to intrinsic nature of crystal growth. As a result, thecomposition of the strained MQW multilayer on the ridge varies slightlyfrom place to place. Utilizing this phenomenon, there can be fabricated,on a plurality of ridges, a corresponding number of optical functionaldevices whose active layers have thicknesses and compositions bothslightly different from each other so that their oscillatingcharacteristics can be controlled within wide ranges.

In controlling the oscillating characteristics of such an opticalfunctional device, major cause of controllability is attributable to thechange in the composition of the MQW layer, particularly when the gapdistance is varied. FIG. 3 illustrates results of calculation ofoscillating characteristic of an In_(1-x) Ga_(x) As/InGaAsP (λg=1.1 μm)crystal depending on the content, x, of Ga, with the thickness of theMQW layer being taken as a parameter. Since the change in the thicknessof the MQW layer formed by the MOVPE method on the ridge when the gapdistance is varied from 1 μm to 10 μm is no greater than 10% regardlessof the ridge width, it is believed that the change in oscillatingwavelength is attributable mainly to the change in the composition ofthe MQW structure.

For example, in the case where the MQW is 30 Å, a decrease in thecontent, x, of Ga of from 0.57 to 0.32 (about 40% decrease) , theoscillating wavelength varies from 1.3 μm to 1.55 μm as indicated by anarrow A. The amount of decrease in the content of Ga is considered to bea reasonable variation amount taking into consideration the differencein the speed of migration between In and Ga on the crystal surface, orproportion of the surface on which such a difference between In and Gaunveils to the total crystal growth surface on the ridge.

On the other hand, the conventional masked selective growth method givesrise to change in oscillating wavelength as indicated by an arrow B inFIG. 3, which shows that the change in oscillating wavelength isattributable to the change in the thickness of the MQW layer rather thanthe change in the composition.

In the above-described case, an optical device which emits light beamsof various wavelengths in a wide range from the same output face can befabricated by growing several kinds of crystals different in compositionin a direction of its cavity. It is suitable that the width of groovesformed in the semiconductor substrate be not smaller than 1 μm,preferably within the range of 1 μm to 10 μm, and that the ridgeexisting between two such grooves have a width of not smaller than 1 μm,preferably within the range of 1 μm to 5 μm, and a height within therange of 1 μm to 5 μm, preferably 1 μm to 3 μm. Preferably, the ridgehas an optical waveguide layer therein.

The above-described growth method allows controlling the composition ofthe active layer by the sizes of gaps and ridges and as a result theoscillating wavelength can be controlled over a wide range of notsmaller than 300 nm.

FIG. 4A shows an example of the structure of a semiconductor lightemitting device which includes a light emitting diode (LD) and a laserdiode (LED) fabricated by the above-described method according to thepresent invention. In FIG. 4A, reference numeral 1 denotes an n-InPsubstrate, 6 is an active layer (for example, a strained MQW layerconsisting of InGaAsP/InGaAs), 6a is an MQW layer in a groove, 7 is ap-InP cladding layer, 8 is an n-InP buried layer, 9 is a p-InP layer,and 10 is a contact layer.

In order to further improve the oscillating characteristics of thesemiconductor light emitting device fabricated by the above-describedmethod, two problems must be solved.

More particularly, for example, when a crystal is grown on a nonplanarsemiconductor substrate 1 having a ridge shape of a ridge width of 1.5μm, a gap distance of 2 μm, and a ridge height of 2 μm using acomposition which gives an oscillating wavelength in a 1.35 μm band in aplanar region of the substrate, the actual composition of the activelayer 6 on the ridge becomes one which corresponds to 1.55 μm band whilethe actual composition of the crystal 6a in the groove corresponds to1.3 μm band. In the case where a semiconductor substrate having the sameridge shape as above is used for growing on it a composition giving anoscillating wavelength of 1.45 μm band or 1.55 μm band in the planarregion of the substrate, the actual composition of the active layer 6 onthe ridge is not varied so greatly and corresponds to an oscillatingwavelength in the vicinity of 1.55 μm for both of the startingcompositions corresponding to 1.45 μm band and 1.55 μm band wile theactual composition of the MQW layer 6a in the groove is varied so as tocorrespond to 1.4 μm band or 1.5 μm band for the starting compositioncorresponding to 1.45 μm or 1.55 μm, respectively.

As stated above, the composition of the active layer 6 on the ridge isconstant, corresponding to 1.55 μm band while the composition of the MQWlayer 6a in the groove is varied considerably.

Analysis of the oscillating characteristics of such a semiconductorlaser diode by a computer simulation revealed that the closer thecomposition of the MQW layer in the groove to the composition of theactive layer on the ridge, the smaller the difference in potential in avertical direction between the both portions, and as a result thereoccurs the more leakage of current into the groove portion to increasethe threshold current.

FIG. 4B illustrates an example of results of simulated oscillation usinga relatively simple model of a semiconductor laser diode having anonplanar substrate of 1.5 μm in ridge width, 2 μm in gap distance, and2 μm in ridge height (groove depth), on which there is grown an MQWlayer having a composition in the groove portion set so as to correspondto 1.3 μm, 1.4 μm or 1.5 μm. In other words, FIG. 4B illustratesdependence of optical output on injected current for varied compositionof the MQW layer in the groove region. In FIG. 4B, the solid lineindicates an ideal state in which there is no crystal defect while thebroken line indicates a state in which crystal quality is insufficient,or there are crystal defects in a density of 10¹² per cm³.

From what is illustrated in FIG. 4B, one will be aware of two problems.First one is that the injected current threshold of the laser diodevaries greatly depending on the composition of the MQW layer in thegroove region. Second one is that the oscillating characteristics variesdepending on crystal quality; the oscillating characteristics obtainedby the current technology is considered to resemble one indicated inbroken line. To improve the oscillating characteristics of a laser diodeas described above, there are needed uniformity of injected currentthreshold and improvement of the quality of a crystal.

Quality of a crystal depends largely on the density of damages occurringduring the patterning of a nonplanar semiconductor substrate underlyingthe crystal although it also depends on the conditions of crystalgrowth. It is considered that the current oscillating characteristicsare attributable to a decrease in crystal quality which occurs when thecrystal growth of an active layer is performed directly on a nonplanarsemiconductor substrate made by a dry process. Accordingly, in order tofurther increase the oscillating characteristic of a current opticaldevice, it is necessary to decrease the occurrence of damages in anonplanar semiconductor substrate inclusive of those damages occurringduring processing before the crystal growth.

The semiconductor light emitting device of the present inventionpreferably has a semiconductor thin film layer formed in theabove-described groove region which layer has a conductivity typedifferent from that of the substrate.

It is also preferred that the active layer be formed on a nonplanarsemiconductor substrate having a ridge shape of an anti-mesa structure.

Further, it is preferred that a semiconductor thin film buffer layer beformed on a nonplanar semiconductor substrate.

The method for the fabrication of the above-described semiconductorlight emitting device includes the steps of forming on a planarsemiconductor substrate a semiconductor thin film protective layerhaving a composition different from that of the semiconductor substrate,and processing the substrate into a nonplanar substrate.

More particularly, in a semiconductor light emitting device having astrained MQW structure formed by MOVPE on a nonplanar semiconductorsubstrate having the above-described specified ridge configuration,improvements are made to introduce a semiconductor current blockinglayer, form a semiconductor buffer thin film layer and introduce ananti-mesa ridge structure.

Formation of the strained MQW thin film on the nonplanar semiconductorsubstrate by the introduction of a semiconductor current blocking thinfilm layer, formation of a semiconductor buffer thin film layer,introduction of an anti-mesa ridge structure enables fabrication of asemiconductor light emitting device having a high quality and a highinjection efficiency. This allows diversified control of amount ofwavelength shift along with control of oscillating wavelength bycontrolling the composition which in turn is controlled by the patternof the ridge and, hence, a highly improved semiconductor light emittingdevice can be realized.

According to a second aspect of the present invention, an integratedoptical circuit is provided which includes a semiconductor laser diode(LD), a first semiconductor detector or photodetector (PD), a secondsemiconductor detector which detects light of a wavelength longer thanthat detected by the first detector, a wavelength filter arrangedbetween the first and second detector, all being connected through anoptical waveguide for integration. This device can be used as a singlewavelength transmitter and as a two-wavelength receiver.

In the integrated optical circuit of the present invention, thesemiconductor multilayer is formed preferably on a ridge formed betweentwo grooves. By so doing, the effective bandgap of the semiconductormultilayer can be varied by adjusting the ridge width, gap distance andridge height. Utilizing this phenomenon, a light emitting layer oractive layer, light absorbing layer and an optical waveguide layer canbe formed on the same semiconductor substrate simultaneously.

First, explanation will be made on the formation of a ridge or ridgesused in the present invention and crystal growth thereon as well aschange in bandgap of the thus grown multilayer.

As shown in FIG. 5A, there is formed on a planar substrate 1 an oxide ornitride layer 2 by pattering. Thereafter, a ridge (or mesa) 3 on bothsides of which there are formed grooves as shown in FIG. 5C by dryetching with a reactive gas ion such as chlorine ion or the like or bywet etching with a hydrochloric acid-containing etchant. The directionof the mesa is the same as in the case of fabrication of ordinarysemiconductor laser diodes, that is, <011> direction on a (100)substrate (i.e., so-called "anti-mesa" direction). Formation of a ridgeor ridges can also be performed utilizing a selective growth. In thiscase, a first mask pattern is formed using an insulator layer 2a whichoccupies a region where a groove 4 is to be formed as shown in FIG. 5B.This selective growth method can also provide a ridge as shown in FIG.5C. In FIG. 5C, ridge width, dw, and gap distance, dg, are defined bydistances on the upper surface of the mesa structure. The height, h, ofthe mesa (ridge) is defined by a difference of height between the uppersurface of the mesa and a flat surface in a bottom region of the groove.On the nonplanar semiconductor substrate having a ridge thus formed isgrown a semiconductor multilayer (MQW structure) by MOVPE.

The MQW structure grown by the above-described method has a compositionwhich varies depending on the ridge width, gap distance and ridge heightand so the bandgap wavelength varies accordingly. FIG. 6A is a graphillustrating shifts of the bandgap wavelengths of MQW structures formedon ridges having the same height of 2 μm, and different widths of 1 μm,2 μm, 3 μm, 4 μm and 5 μm, respectively, with the gap distance beingvaried from 1 μm to 10 μm from the bandgap wavelengths of MQW structuressimultaneously grown on a planar region of the substrate. FIG. 6B is agraph illustrating shifts of the bandgap wavelengths of MQW structuresformed on ridges of the same ride width and three different heights of1.2 μm, 1.6 μm and 2.0 μm, with the gap distance being variedcontinuously from the bandgap wavelengths of MQW structuressimultaneously grown on a planar region of the substrate. As shown inFIGS. 6A and 6B, shift of bandgap wavelength till 300 nm can be achievedby growing a MQW structure on a nonplanar semiconductor substrate. Thenarrower the ridge width or gap distance, the longer wavelength side thebandgap wavelength is shifted.

As the MQW structure, there can be used any combination of compositionssuch as InGaAs/InGaAsP, InAsP/InGaAsP, InGaAsP/InGaAsP, InGaAs/InP,InGaAs/InAlAs, InGaAs/InGaAlAs, etc.

According to a third aspect of the present invention, there is providedan optical device in the form of a spot size converter makes most ofchanges in the composition and thickness of an epitaxial layer grown ona ridged substrate depending on the shape of the ridge-like structure orwaveguide. The changes in the composition or thickness of theepitaxially grown layer results in changes in the bandgap wavelength andrefractive index.

Here, the "ridge-like waveguide" consists of grooves and a ridge formedbetween the grooves. Hence, the sizes of the ridge-like waveguideinclude not only the ridge width and ridge height but also the width ofthe groove (gap distance).

FIGS. 7 and 8 illustrate variation of PL (photoluminescence) peakwavelength of a spot size converter which has a MQW structure consistingof an InGaAs layer of 17 Å thick and a 1.1 μm (composition)-InGaAsPlayer of 150 Å thick. FIG. 7 illustrates dependence of wavelength shifton the gap distance, dg, using the ridge width, dw, as a parameter, whenthe ridge height is 2.0 μm. PL peak wavelength on a planar substratewithout any ridge is about 1.27 μm. FIGS. 7 and 8 show that when thesize or sizes of the ridge structure is or are varied in a direction oflight transmission, the epitaxial layer grown on the ridge has a bandgapwavelength which is varied in a direction of light transmission withconcomitant change in refractive index. Therefore, an optical waveguidewhich includes such a layer formed as a core has refractive indexdistributions of the core and cladding, respectively, which differ atboth end faces of the waveguide. As a result, there can be readilyrealized an optical device which includes waveguides of different spotsizes, respectively.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereinbelow, the present invention will be described in more detail byexamples with reference to the drawings. However, the present inventionshould not be construed as being limited thereto.

Embodiment 1 Light Emitting Device

The instant embodiment relates to a light emitting device (lightemitting diode, LD, and laser diode, LD) fabricated by MOVPE growing ofa crystal on a nonplanar InP substrate whose ridge width, ridge distance(gap distance) and ridge height were varied. The light emitting devicehad a oscillating wavelength region near a region of 1.3 μm to 1.65 μm.

FIGS. 9A through 9E illustrate a process for fabricating the lightemitting device of the instant embodiment.

First, as shown in FIG. 9A, a mask pattern for forming a ridge ofpredetermined sizes was formed on an InP substrate 1 with an oxide ornitride layer 2.

Next, as shown in FIG. 9B, grooves 4 for forming a ridge 3 were formedby dry etching with a reactive gas ions, for example, halogen ions suchas chlorine ions, bromine ions, hydrocarbon ions such as those derivedfrom methane, ethane, etc., ion milling with inert gas ions such as Arions, or wet etching with an etchant such as chlorine, sulfuric acid,bromine or the like. The angle of the formation of the ridge 3 was in adirection of <011> on an InP (100) substrate (so-called "anti-mesa"direction). As will be described later on, it has empirically revealedthat this direction gives the largest shift.

The ridge 3 may be formed by a selective growth method. In this case,the mask pattern is reversed to that used in the formation of ridges byetching. That is, as shown in FIG. 9B, masks 2a were placed on regionsin which grooves were to be formed. The selective growth method alsogave the ridge 3 as shown in FIG. 9C. The ridge 3 may also be formed bycrystal growth of an optical waveguide layer of InGaAsP to a thicknessof about 1,000 Å on a semiconductor substrate by an MOVPE method detailsof which will be described later on, followed by formation of ridgednonplanar substrate as shown in FIG. 9C. In this case, freedom increasesin the design of optical functional devices. In FIG. 9C, the ridgewidth, dw, and the gap distance, dg, were measured in terms of distanceon the upper surface of the mesa structure, and the height of the ridge3 was defined by a difference between the upper surface of the ridge anda flat surface of the bottom portion of the groove, taking intoconsideration the possibility that the shape of the ridge 3 was notupright.

Subsequently, a semiconductor multilayer of a laser diode structure wasformed on the nonplanar InP substrate 1 having the ridge 3 by an MOVPEmethod.

The semiconductor multilayer was grown in a low pressure vertical MOVPEgrowth furnace at a pressure of 70 Torr and a growth temperature of 630°C. As the source material, there were used TMI (trimethylindium) , TEG(triethylgallium), AsH₃ (arsine), and PH₃ (phosphine).

The semiconductor multilayer consisted of an n-InP buffer layer 5, a 4-to 6-period InGaAsP strained MQW active layer of InGaAs (17 Å) /InGaAsP(150 Å) (lg=1.1 μm) and InGaAsP optical waveguide layer 6, a p-InPcladding layer 7.

Further, an upper portion of the ridge was masked and an n-InP buriedlayer 8 was grown as shown in FIG. 9E. Thereafter, the mask was removedand a p-InP buried layer 9 and a p-InGaAsP contact layer 10 were grownoverall (FIG. E). The growth steps shown in FIGS. 9D and 9E may beperformed as a through process by controlling growth conditions such asgrowth temperature, doses of dopants, etc. or the ridge width, dw.

Finally, as shown in FIG. 9F, a p-type electrode 11 was formed on thep-InGaAsP contact layer 10 as the uppermost multilayer on the ridge, andan n-type electrode 12 was formed on the InP substrate 1. Afteralloying, devices were fabricated by cleavage or separation ofelectrodes. Injection of current in the resulting device caused laserbeam emission in a direction indicated by an arrow in FIG. 9F.

In the above-described fabrication process, a light emitting device of aburied heterostructure was realized on an InP substrate. When currentwas injected into the light emitting device, there was observed lightemission from an end face of the ridge 3.

FIGS. 10A through 10C illustrate optical characteristics of the lightemitting device of the present invention in comparison with theconventional light emitting device in which the active layer is formedon a nonplanar semiconductor substrate.

More particularly, FIGS. 10A through 10C illustrate shifts in theoscillating wavelength of a MQW layer of In_(1-x) GaxAs/InGaAsP (lg=1.1μm) of 17 Å thick which was formed on a ridged substrate and had atension of 0.5%. For comparison, a light emitting device was fabricatedby growing on a planar substrate a crystal of MQW layer similarly to thepresent invention, dry etching the active layer to a predeterminedwidth, and regrowing a buried semiconductor thin film layer. Thecomparative light emission had an oscillating wavelength of 1.27 μm.

FIG. 10A illustrates dependence of wavelength shift on the ridge width,dw, ranging from 1 μm to 6 μm, using the gap distance, dg, as aparameter, when the ridge height is 2.0 μm. As will be apparent fromFIG. 10A, at a gap distance of not greater than 3 μm (dg<3 μm), shift ofthe oscillating wavelength increases considerably according as the ridgewidth, dw, decreases. On the contrary, shift of the oscillatingwavelength is little at a ridge width of not smaller than 4 μm.

FIG. 10B illustrates dependence of wavelength shift on the gap distance,dg, ranging from 1 μm to 10 μm, using the ridge width, dw, as aparameter, when the ridge height is 2.0 μm. As will be apparent fromFIG. 10B, shift of the oscillating wavelength increases considerablyaccording as the gap distance, dg, decreases when the ridge width, dw,is not greater than 2 μm (dw≦2 μm). This variation is remarkable whenthe ridge width is not greater than 6 μm.

FIG. 10C illustrates dependence of wavelength shift on the ridge height,h, ranging from 1.2 μm to 2.2 μm, using the ridge width, dw, as aparameter, when the gap distance, dg, is a constant value of 1.5 μm. Aswill be apparent from FIG. 10C, shift of the oscillating wavelengthincreases considerably according as the ridge height, h, increases. Fromthe characteristics illustrated in FIGS. 10A through 10C, it can be seenthat the shift of oscillating wavelength is very large when the ridgehas minute sizes, i.e., a ridge width, dw, a ridge height, h, and a gapdistance, dg, each not greater than 10 μm, and in addition that theoscillating wavelength can be controlled by processing the ridge, etc.The oscillating wavelength shifts at a longer wavelength side with asmaller ridge width or a smaller gap distance. According to the presentinvention, there can be realized a light emitting device having amaximum oscillating wavelength of 1.6 μm in contrast to the conventionallight emitting device having an oscillating wavelength of 1.3 μm using aplanar substrate. This means a shift of 300 nm can be achieved by thepresent invention.

To confirm what is a major factor of the shift of the oscillatingwavelength according to the present invention, an InGaAs (λg=1.1 μm) MQWlayer was grown on a ridged substrate, and dependence of the thicknessof the MQW layer on the ridge width, dw, gap distance, dg, or the ridgeheight, h, of the substrate was examined. FIGS. 11A through 11Cillustrate the results in such a manner that they correspond torespective sizes in the embodiments. FIG. 11A is a graph illustratingdependence of the thickness of the MQW layer on the ridge width. Thephenomenon that the ridge width, dw, abruptly increases at a ridge widthof not greater than 1.5 μm is believed to reflect the migration ofcomponent atoms, particularly in, on the crystal surface.

FIG. 11B is a graph illustrating dependence of the thickness of the MQWlayer on the gap distance, dg. On the other hand, FIG. 11C illustratesdependence of the thickness of the MQW layer on the ridge height, h.Both FIGS. 11B and 11C show less variation in the thickness of the MQWlayer. From the characteristics illustrated in FIGS. 11A through 11C, itcan be seen that although the thickness of the MQW increases slightlywithin the ranges of the ridge width, dw, ridge height, h, and gapdistance, dg, each not greater than 10 μm, used in the presentinvention, the major factor that changes the shift of the oscillatingwavelength greatly is a change in the composition of the MQW layer dueto effects of the shape of the ridge.

What is described above is an example in which the strained MQW layerconsists of InGaAs/InGaAsP. Also, there may be used other III-V groupMQW materials that can grow on an InP substrate, for example,InGaAsP/InGaAsP, InGaAs/InP, InAsP/InAlAs, InGaAs/InGaAlAs, etc. Theabove-described control of the oscillating wavelength can be achieved ina shorter wavelength region (from 0.8 μm to near 1.1 μm) by growing anMQW structure of GaAs/AlGaAs, InGaAs/AlGaAs, etc. on a GaAs substrate.

Embodiment 2 DFB Laser Diode

The instant embodiment relates to fabrication of a DFB laser diode whichhas an improved controllability of the oscillating wavelength of thelight emitting device described in Embodiment 1, and which oscillates ina single spectrum.

FIGS. 12A through 12C show formation of diffractive gratings on a ridgedsubstrate for fabricating a DFB laser diode according to the instantembodiment.

FIG. 12A is a perspective view showing a semiconductor substrate havinga ridge on which diffractive gratings of a predetermined period areformed. The ridged substrate was formed with diffractive gratings 13 ofa predetermined period by electron beam lithography or laser holographyin a region where a ridge is to be formed, followed by forming a ridge3. Subsequent processes such as formation of a semiconductor MQW layerand fabrication of a light emitting device were the same as inEmbodiment 1.

FIG. 12B is a perspective view showing a semiconductor substrate forfabricating a laser diode of the present invention. After asemiconductor optical waveguide 14 was formed on a substrate 1,diffractive gratings 13 having a predetermined period were formed byelectron beam lithography in a region where a ridge was to be formed.Then, the so-processed substrate 1 was formed with a ridge 3 similarlyto the substrate as shown in FIG. 12A. Subsequent processes such asformation of a semiconductor MQW layer and fabrication of a laser diodewere the same as in Embodiment 1 except that the step of forming anoptical waveguide layer was unnecessary.

FIG. 12C is a perspective view showing an example of a ridgedsemiconductor substrate for fabricating a laser diode of the presentinvention. The ridged substrate shown in FIG. 12C has diffractivegratings 15 on side faces of a ridge 3. This construction was obtainedby the steps of processing by electron beam lithography a mask forforming a ridge to provide the mask with diffractive gratings of apredetermined period, forming a ridge 3 and simultaneously formingdiffractive gratings 15 on a side face of the ridge 3. Subsequentprocesses such as formation of a semiconductor MQW layer and fabricationof a laser diode were the same as in Embodiment 1.

In the instant embodiment, the laser diode operated at a singlelongitudinal mode upon injection of current into the laser to causelight emission.

Embodiment 3 Gap Distance-Modulated Laser Diode array

The instant embodiment relates to fabrication of a semiconductor laserdiode (LD) array having constant ridge width, dw, and constant ridgeheight of a groove 4, with the gap distance, dg, being varied in fourways. FIGS. 13A through 13E show a fabrication process for fabricatingan LD diode array according to the instant embodiment.

First, a stripe pattern made of an oxide or nitride layer 2 was formedon an n-InP (100) substrate 1 by photolithography as shown in FIG. 13A.Then, the substrate was dry etched with chlorine gas to form a nonplanarsubstrate having ridges 3 and grooves 4 of a gap distance-modulated typeas shown in FIG. 13B. In this case, the ridges 3 having a ridge width,dw, of 1.5 μm were formed in a direction of <011> (so-called "anti-mesa"direction). The ridge height was 2 μm. The gap distance, dg, was variedserially from 1.5 μm on the leftmost side, toward right hand side 1.3μm, 3.0 μm, 3.5 μm, and 4.0 μm on the rightmost side.

Subsequently, there were formed on the n-InP substrate 1 an InP bufferlayer 5, an active layer and optical waveguide layer 6 including anInGaAsP optical waveguide and a strained MQW active layer consisting of5-periods of InGaP well (20 Å)/InGaAsP barrier (150 Å), and a p-InPcladding layer 7, as shown in FIG. 13C. A semiconductor multilayerconsisting of the semiconductor layers 5, 6 and 7 was grown by an MOVPEmethod at 630° C. and at a pressure of 0.1 atm using TMI, TMG, AsH₃, PH₃as source gases for fabricating a semiconductor and selenium hydride anddiethylzinc as doping gases.

Further, an n-InP layer 8 and a p-InP layer 9 for blocking current wereformed by an MOVPE method, and then a p-InGaAsP contact layer 10 wasformed on the layer 10 as shown in FIG. 13D. The crystal growth stepsshown in FIGS. 13C and 13D may be performed in one time by controllingthe growth conditions. However, in order to exactly control thethickness of the buried layer and doping, the growth may be performed intwo or three times dividedly.

Subsequently, as shown in FIG. 13E, p-type electrodes 11 and n-typeelectrodes 12 were formed on the both sides of the substrate, andisolation grooves 16 were formed to electrically isolate the device in atransverse direction. The device thus fabricated was mounted on a heatsink 17 and leads 18 were bonded to the electrodes 11. Thereafter,currents I₁, I₂, I₃ and I₄ were injected into the ridges 3. As a result,there was observed emission of laser beams A₁, A₂, A₃ and A₄ in adirection indicated by arrows shown in FIG. 13E.

The oscillating wavelength corresponded well to the results ofmeasurements of photoluminescence, which confirmed that there wasobtained light emission of high directivity corresponding to the changein composition. The oscillating spectrum on a planar substrate observedby photoluminescence had a peak at 1.3 μm whereas laser beams A₁, A₂, A₃and A₄ from the respective end faces of the ridges separated by twoadjacent grooves at a gap distance of 1.5 μm, 3.0 μm, 3.5 μm or 4.0 μm,respectively, had peaks at 1.55 μm, 1.5 μm, 1.45 μm or 1.4 μm,respectively, as shown in FIG. 14.

Embodiment 4 Gap Distance-Modulated Laser Diode Array with DiffractiveGratings

According to the instant embodiment, diffractive gratings wereintroduced into the construction of the LD array according to Embodiment3 above in order to improve the controllability of oscillatingwavelength and make the device oscillate in a single spectrum.

FIGS. 15A through 15E show a fabrication process for fabricating gapdistance-modulated LD array according to the present invention.

First, diffractive gratings 13 were formed on an InP substrate 1 byelectron beam lithography as shown in FIG. 15A. The diffractive gratingshad a width of 5 μm and various periods of 2,400 Å, 2,300 Å, 2,200 Å,and 2,100 Å, respectively, from right hand side toward left hand side.Sometimes, before the diffractive gratings 13 were formed on the InPsubstrate, an optical waveguide layer, for example, an InGaAsP layer,had been formed in advance by crystal growth. In this case, thecorresponding step of growing such an optical waveguide in thesubsequent process was omitted.

Next, the same procedure as in Embodiment 3 were repeated. That is,after a stripe pattern consisting of an oxide or nitride layer 2 wasformed as shown in FIG. 15B, the substrate was dry etched to give anonplanar substrate having ridges 3 and grooves 4 as shown in FIG. 15C.Further, a semiconductor multilayer consisting of the active layer andoptical waveguide 6 and the cladding layer 7 were grown on the nonplanarsubstrate 1 as shown in FIG. 15D, and the buried layers 8, 9 and 10 weregrown on the layer 7 as shown in FIG. 15E. Then, p-type electrodes 11were formed on the buried layer 10 while n-type electrodes 12 wereformed on the substrate 1, the resulting structure being mounted on aheat sink 17, followed by attaching leads 18 to the electrodes 11.Isolation grooves 16 were formed in the buried layer 10 to electricallyisolate the electrodes 11. When currents I₁, I₂, I₃ and I₄ were injectedinto the ridges 3, there was observed emission of laser beams A₁, A₂, A₃and A₄ in a direction indicated by arrows shown in FIG. 15F. As shown inFIG. 16, there were obtained 4-wavelength integrated emissions A₁, A₂,A₃ and A₄ each in a single mode.

Embodiment 5 Ridge Width-Modulated Laser diode array

The instant embodiment relates to a fabrication of an LD array having aridge 3 that has a constant height, h, and a constant gap distance, dg,with the width, dw, being varied in four ways. FIGS. 17A through 17Eshow a fabrication process for fabricating an LD array according to theinstant embodiment.

A stripe pattern made of an oxide or nitride layer 2 was formed on ann-InP (100) substrate 1 by photolithography as shown in FIG. 17A. Thesubstrate was dry etched with chlorine gas to form a nonplanar substratewith ridges 3 and grooves 4 of a ridge width-modulated type as shown inFIG. 17B. Here, the ridge height, h, and the gap distance, dg, werefixed to 2 μm and 1.5 μm, respectively, and the ridge width, dw, wasvaried from 1.5 μm at the rightmost side toward 2.0 μm, 2.5 μm and 3.0μm at the leftmost side.

Subsequently, there was formed on the nonplanar substrate a crystalhaving an active layer 6 of a strained MQW structure consisting of5-periods of InGaP well (20 Å)/InGaAsP barrier (150 Å) together with thesemiconductor layers 5 and 7 in the same manner as in Embodiment 3, asshown in FIG. 17C.

Further, embedding regrowth was performed to form buried layers 8, 9 and10 in the same manner as in Embodiment 3 to obtain a structure as shownin FIG. 17D. Also, formation of electrodes 11 and 12 and subsequentprocedure including mounting of the device on a heat sink 17 wereperformed in the same manner as in Embodiment 3 to finally obtain afour-wavelength integrated laser diode array as shown in FIG. 17E. Thishad oscillating characteristics similar to those illustrated in FIG. 16.That is, oscillating spectrum on a planar substrate was observed to havea peak at 1.3 μm while the peak emission from the active layer on theridge varied from 1.55 μm, to 1.5 μm, 1.45 μm, and 1.4 μm according asthe ridge width increases from 1.5 μm to 2.0 μm, 2.5 μm, and 3.0 μm.

Embodiments 6 Detector

The instant embodiment relates to a detector having the same structureas the light emitting device of Embodiment 1 except that the activelayer in the structure of Embodiment 1 was replaced by a detectinglayer. This detector was fabricated by the same process as inEmbodiment 1. The detector of the instant embodiment like light emittingdevices responded to light having a wavelength corresponding to thecomposition of the detecting layer formed on the ridge. Therefore,optical response was able to be detected by inputting light in adirection opposite to that of the arrow in FIG. 9F and detecting currentgenerated in the electrode 11. For example, a detector having a 1.3μm-composition did not respond to a 1.55 μm beam inputted through an endface while when both 1.3 μm and 1.55 μm light beams were inputtedsimultaneously, the detector responded to a light beam of 1.3 μm alone.

Embodiment 7 Integrated Optical Device

Various other optical functional devices can be realized by fabricationprocesses similar to the process shown in FIGS. 9A through 9F. Forexample, there can be fabricated optical waveguides having differentoptical characteristics depending on different compositions, opticalmodulators or optical switches operating within specified wavelengthregions, wide-band semiconductor amplifiers, etc. Simultaneousfabrication of these on one semiconductor substrate results in therealization of integrated optical devices having high interdevicecoupling efficiencies.

More concretely, there can be cited as an example a monolithicintegrated optical circuit for use in receivers in coherentcommunication systems. As shown in FIG. 18, this integrated circuitincludes a wavelength-tunable multielectrode DFB laser 30 as a localoscillation light source, a directional coupler-type 3 dB coupler 31, awaveguide-type PIN photodetector 32, a butt-joint portion 33, andoptical waveguides for transmitting optical signals. By combination of4- or 5-times of crystal growths and high processing techniques, therehave already been realized a device which affords fundamental operations(Takeuchi, et al., Denshi Joho Tsushin Gakkai Ronbunshi, C-f1, Vol.J73-J-1, No. 5, pp360-367, May 1990) (The Institute of Electronics,Information and Communication Engineers, transactions, C-f1, Vol.J73-J-1, No. 5, pp360-367, May 1990).

However, the above-described device has insufficient characteristics,not to speak of optical coupling efficiency, because of repetition ofthe process many times. According to the present invention, theabove-described integrated optical circuit can be realized by only onegrowing operation if only a mask pattern is formed on the substrate inthe beginning, and the product has excellent characteristics to beginwith coupling efficiency. This makes crystal growth and the processsimplified so that fabrication cost can be reduced greatly.

As described concretely by Embodiments 1 to 7 above, the presentinvention enables one to relatively easily realize optical devicesincluding a semiconductor substrate on which a plurality of opticalfunctional devices are arranged whose optical characteristics differslightly from each other, or on which optical functional devices areintegrated at high density. This makes a rapid progress in opticalcommunication.

Embodiment 8 Gap Distance-Modulated Superluminescent Photodiode (SLD)

FIGS. 19A through 19E show a process for fabricating a gapdistance-modulated superluminescent photodiode (SLD) according to theinstant embodiment. In this embodiment, the ridge width and ridge heightwere made constant, and the gap distance was varied in five ways.

As shown in FIG. 19A, a stripe pattern made of an oxide or nitride layer2 was formed on an n-InP (100) substrate 1 by photolithography. Thispattern was constructed by a stripe which corresponded to a ridge to beformed and two mask pieces sandwiching the stripe therebetween. Themasks were in the form of a plateau stepped along the length of thestripe. When an optical waveguide, for example, an InGaAsP layer, wasgrown on the n-InP substrate before the patterning, the correspondinggrowth step in the subsequent crystal growth was able to be omitted.

Next, the substrate was dry etched with chlorine gas to form a nonplanarsubstrate with ridges 3 and grooves 4 of a gap distance-modulated typeas shown in FIG. 19B. Here, the ridge 3 was formed in a direction of<011> (so-called "anti-mesa" direction), and the ridge width, dw, andthe ridge height, h, were set to 1.5 μm and 2 μm, respectively. The gapdistance, dg, was set, at every 300 μm along the length of the ridge, to1.5 μm, 3.0 μm, 3.5 μm and 4.0 μm, and 10.0 μm.

Subsequently, there were formed, on the nonplanar substrate. 1, an InPbuffer layer 5, an active layer 6 having an InGaAsP waveguide layer anda strained MQW structure consisting of 5-periods of InGaP well (20Å)/InGaAsP barrier (150 Å), and a p-InP layer 7, as shown in FIG. 19C.These-layers were grown by MOVPE using TMI, TMG, AsH₃, and PH₃ as sourcegases for fabricating semiconductors and selenium hydride anddiethylzinc as doping gases at 630° C. and at 0.1 atm.

The spectrum on a planar substrate observed by a photoluminescencemethod had a peak at 1.3 μm while the peak emission from the activelayer on the ridge whose adjacent grooves was of a gap distance of 1.5μm, 3.0 μm, 3.5 μm, 4.0 μm or 10.0 μm varied from 1.55 μm, to 1.5 μm,1.45 μm, 1.4 μm, or 1.35 μm, respectively.

Further, using MOVPE, a p-InP cladding layer 7' was overgrown on thep-InP cladding layer 7, and then an n-InP layer 8 and a p-InP layer 9were grown on the cladding layer 7 for blocking current. Thereafter, ap-InGaAsP contact layer 10 was formed as shown in FIG. 19D. The crystalgrowth steps shown in FIGS. 19C and 19D may be performed in one time bycontrolling the growth conditions. However, in order to exactly controlthe thickness of the buried layer and doping, the growth may beperformed in two or three times dividedly.

Thereafter, p-electrodes 11 and n-electrodes 12 were formed on the upperand lower faces of the substrate, and isolation grooves 16a forisolating individual portions which vary in the composition on theridges were provided as shown in FIG. 19E.

The device thus fabricated was mounted on a heat sink 17, and wiring 18was bonded. Upon injection of currents I₁ through I₅, highly directionallight emissions dependent on the injected currents were observed in adirection shown by an arrow A in FIG. 19E. The emitted light beamscorresponded to respective compositions of the active layers on theridges, and also corresponded well to results of photoluminescencemeasurement indicated by thin solid lines A₁ through A₅ in FIG. 20.

When current was applied to all of the electrodes simultaneously, thedevice oscillated in a wide band ranging from 1.3 μm to 1.6 μm as shownin a thick solid line A in FIG. 20. Then, another emission was observedas shown in a thick broken line B in FIG. 20 in a direction indicated byan arrow B which is opposite to the direction indicated by the arrow Ain FIG. 19. This is because light with a relatively short wavelengthemitted on the front face is absorbed in a region opposite thereto.

Embodiment 9 Ridge Width-Modulated SLD

The instant embodiment relates to a ridge width-modulated SLD, and FIGS.21A through 21E illustrate a process for fabricating such a ridgewidth-modulated SLD. In this embodiment, the groove width (gap distance)and groove depth (ridge height) were kept constant while the ridge widthwas varied in five ways.

As shown in FIG. 21A, a stripe pattern made of an oxide or nitride layer2 was formed on an n-InP (100) substrate 1 by photolithography. Thispattern was constructed by a polygonal stripe portion which included anexpected ridge portion and two mask pieces sandwiching the stripeportion therebetween. The masks were in the form of a plateau stepped inwidth along the length of the stripe.

Next, the substrate was dry etched in the same manner as in Embodiment 8to form a nonplanar substrate with a ridge 3 and grooves 4 of a gapdistance-modulated type as shown in FIG. 21B. Here, the ridge 3 wasformed in a direction of <011> (so-called "anti-mesa" direction). Theridge 3 had 5 kinds of ridge widths, dw, differing at every 300 μm alongthe length of the ridge, i.e., 1 μm, 2 μm, 3 μm, 4.0 μm, and 10.0 μm.The height of the ridge, h, was 2 μm, and the gap distance, dg, was 2.5μm.

Subsequently, the procedures of Embodiment 8 were repeated to grow, onthe nonplanar substrate 1, an InP buffer layer 5, an active layer 6including an InGaAsP waveguide layer and a strained MQW structureconsisting of 5-periods of InGaP well (20 Å)/InGaAsP barrier (150 Å),and a p-InP layer 7, as shown in FIG. 21C.

The oscillating spectrum of the crystal grown on the ridge was measuredby photoluminescence. The crystal had different peak wavelengths fromcrystal portion to crystal portion corresponding to different ridgewidths. Thus, the peak wavelengths of 1.55 μm, 1.5 μm, 1.45 μm, 1.4 μm,and 1.35 μm corresponded to ridge widths of 1 μm, 2 μm, 3 μm, 4.0 μm,and 10.0 μm.

Thereafter, in order to obtain a buried heterostructure, an oxide ornitride layer 2' was again formed as a mask as shown in FIG. 21D,followed by dry etching to obtain a structure as shown in FIG. 21E.Embedding regrowth and formation of electrodes were performed in thesame manner as in Embodiment 8 to obtain structures similar to thoseshown in FIGS. 19D and 19E, respectively. The optical device thusfabricated had oscillating characteristics similar to that illustratedin FIG. 20.

Embodiment 10 Ridge Height-Modulated SLD

The instant embodiment relates to a ridge height-modulated SLD. FIGS.22A through 22E illustrate a fabrication process for fabricating such aridge height-modulated SLD. In the instant embodiment, the ridge widthand groove width (gap distance) were kept constant while the ridgeheight (groove depth) was varied in five ways.

First, as shown in FIG. 22A, a stripe pattern made of an oxide ornitride layer 2 was formed on an n-InP (100) substrate 1 byphotolithography. This pattern comprises two plateau-shaped mask pieces,formed on the substrate 1 at a predetermined distance from each other sothat a ridge was able to be formed therebetween. Each of the mask pieces2 had portions stepped in width on the outer side as shown in FIG. 22A.

Next, an n-InP layer was grown selectively to form a nonplanar substratehaving a ridge sandwiched by two grooves with different groove widths orgap distances along the length of the ridge, as shown in FIG. 22B. Theridge thus formed had different heights along its length and, hence,stepped in height as shown in FIG. 22B, since growth rate varieddepending on the width of the mask.

Here, the ridge 3 was formed in a direction of <011> (so-called"anti-mesa" direction). The ridge 3 had five kinds of ridge heights, h,differing at every 300 μm along the length of the ridge, i.e., 1.2 μm,1.4 μm, 1.6 μm, 1.8 μm, and 2.0 μm. The width of the ridge, dw, was 1.5μm, and the gap distance, dg, corresponded to the widths of the mask.

Subsequently, in order to make the gap distance uniform, the oxide ornitride layer 2 was formed again as a mask as shown in FIG. 22C, and thesubstrate was dry etched to obtain a nonplanar substrate which differedonly in ridge height, h, and had constant ridge width, dw, and gapdistance, dg, along the length of the ridge as shown in FIG. 22D. Thegap distance was adjusted to 2.5 μm uniformly.

Then, the procedures of Embodiment 8 were repeated to grow, on thenonplanar substrate 1, an InP buffer layer 5, an active layer 6including an InGaAsP waveguide layer and a strained MQW structureconsisting of 5-periods of InGaP well (20 Å)/InGaAsP barrier (150 Å),and a p-InP cladding layer 7, as shown in FIG. 22E.

The spectrum of the crystal grown on the ridge was measured byphotoluminescence. The crystal had different peak wavelengths fromcrystal portion to crystal portion corresponding to different ridgeheights. That is, there were obtained peak wavelengths of 1.35 μm, 1.4μm, 1.45 μm, 1.5 μm, and 1.55 μm which corresponded to ridge heights of1.35 μm, 1.4 μm, 1.45 μm, 1.5 μm, and 1.55 μm.

Further, embedding regrowth was performed in the same manner as inEmbodiment 8 to give a structure similar to that shown in FIG. 19D,followed by formation of electrodes in the same manner as in Embodiment8 to give an optical device similar to that shown in FIG. 19E. Theoptical device thus fabricated had oscillating characteristics similarto that illustrated in FIG. 20.

Variation

FIGS. 23A through 23C illustrate a variation of a process forfabricating a ridge height-modulated SLD according to Embodiment 10.

As shown in FIG. 23A, a mask 2 was so that two mask pieces were arrangedin a relation inside out to the situation shown in FIG. 22A. Afterselective growth, a nonplanar substrate was fabricated whose ridge hadvarious widths and heights along its length as shown in FIG. 23B. Inorder to make the ridge width uniform, an oxide or nitride layer 2' wasformed again as shown in FIG. 23C, and the substrate was dry etched togive a nonplanar substrate of which only the ridge height varied,similar to the nonplanar substrate shown in FIG. 22D. The device thusfabricated had oscillating characteristics similar to that shown in FIG.20.

Further, a ridge height-modulated SLD was fabricated in the same manneras in Embodiment 10 to have a structure similar to that shown in FIG.22E. The device thus obtained had oscillating characteristics similar tothat shown in FIG. 20.

As described concretely in Embodiments 8 to 10 highly integrated SLD canbe realized by the present invention. Therefore, there is expected rapidprogress in the development of measurement or evaluation methods makingmost of the advantage of wide-band oscillating wavelengthcharacteristics.

Embodiment 11 Semiconductor Light Emitting Device with a CurrentBlocking Layer

FIGS. 24A through 24E illustrate a process for fabricating a lightemitting device which has a current blocking layer in grooves of anonplanar semiconductor substrate.

First, oxide layers 2, 2b (same composition) were formed on an n-InPsubstrate 1 for etching in a predetermined pattern for preparing anonplanar semiconductor substrate as shown in FIG. 24A. Then, the n-InPsubstrate 1 was dry etched by reactive ion etching with chlorine gas sothat grooves 4 were formed therein. Thereafter, the oxide layer 2 wasremoved while the oxide layer 2b on the ridge was left as was. Thus,there was obtained a nonplanar substrate having a ridge width, dw, aridge height, h, and a gap distance, dg, as shown in FIG. 24B.

On this nonplanar substrate was a p-InP layer 19 was grown to athickness of from about 0.2 μm to 0.3 μm by MOVPE at a pressure of 70Torr and at a substrate temperature of from about 600° C. to 700° C.with feeding TMI, PH₃, and diethylzinc (Zn (C₂ H₅)₂) as a p-type dopantas shown in FIG. 24C. The thin film layer thus grown which had aconductivity type different from that of the nonplanar substrate will bereferred to hereinafter to "current blocking layer".

Next, the oxide layer 2b on the ridge 3 was removed and there wereformed an MQW active layer 6 and an in-groove MQW layer 6a, both layershaving a composition of InGaAsP/InGaAs. These layers 6 and 6a wereformed by feeding predetermined amounts (flow rates) of TEGa, AsH₃, TMI,and PH₃ in a manner such that 1.1 μm-InGaAsP thin film of 150 Å thickand InGaAs thin film of 20 Å were grown alternately in, for example, 4to 6 periods, to form an MQW structure. Subsequently, a p-InP claddinglayer 7, an n-InP buried layer 8, a p-InP layer 9, a p-InGaAsP contactlayer 10 were grown in this order to obtain a buried heterostructure asshown in FIG. 24D.

The process according to the instant embodiment enables controllingoscillating wavelength of the fabricated light emitting device to withinthe range of from 1.3 μm to 1.6 μm depending on the shape factors of thesemiconductor substrate, i.e., ridge width, dw, gap distance, dg, andridge height, h. In the process of the instant embodiment, since thep-InP current blocking layer 19 was to be formed first, adjustment ofthe ridge height was necessary. It was sufficient to adjust the ridgeheight by making the ridge height, i.e., groove depth, prior to thegrowth of the current blocking layer (cf. FIG. 24B) larger than whatshould be obtained finally by a decrease in the groove depth by thegrowth of a current blocking layer, i.e., by the thickness of thecurrent blocking layer to be grown. Thereafter, a p-electrode 11 wasformed on a portion of the contact layer 10 just above the ridge 3 whilean n-electrode 13 was formed on the side of the semiconductor substrateto fabricate a light emitting diode chip as shown in FIG. 24E.

FIG. 25 is a graph which illustrates the oscillating characteristics ofthe light emitting device fabricated by the above-described process insolid line. For comparison, the oscillating characteristics of a similarlight emitting device having no current blocking layer was indicated inbroken line. In the instant embodiment, provision of a current blockinglayer resulted in decrease in threshold of injected current and highlinearity to increase oscillating efficiency and output. Hence, it wasconfirmed that the introduction of p-InP current blocking layer 19improved the oscillating characteristics considerably.

Embodiment 12 Light Emitting Device with a Buffer Layer

FIGS. 26A to 26D illustrate a fabrication process for fabricating alight emitting device having an n-InP thin film layer as a buffer layergrown on a nonplanar semiconductor substrate in order to avoid theoccurrence of a damaged layer during the preparing the nonplanarsemiconductor substrate so that the oscillating characteristics can beimproved. To examine influences of the presence of a buffer layer andthe shape of the ridge, two types of light emitting devices werefabricated which had buffer layers of normal-mesa and anti-mesastructures, respectively.

First, an oxide layer 2 was formed on an n-InP substrate 1 for etchingin a predetermined pattern as shown in FIG. 26A.

Next, the substrate 1 was dry etched by reactive ion etching (RIE) with,for example, chlorine gas to form grooves 4 so that the n-InP substrate1 was able to have a ridge 3 of a normal-mesa structure to obtain anormal-mesa type nonplanar semiconductor substrate (FIG. 26B). On theother hand, another n-InP substrate 1 having the oxide layer 2 was wetetched with bromine-containing etchant, for example, an aqueous solutionof hydrobromic acid (3HBr+H₂ O) to fabricate an anti-mesa type nonplanarsemiconductor substrate having a ridge 3 of an anti-mesa structure (FIG.26C). Thereafter, the oxide layer 2 was removed from the both types ofthe semiconductor substrates.

Then, a buffer layer 5 was grown to a thickness of about 0.1 μm to 1.0μm. The growth was performed by MOVPE at a pressure of 70 Torr and asubstrate temperature within the range of from about 600° C. to 700° C.,with feeding as the source material TMI and PH₃ as well as seleniumhydride (H₂ Se) as an n-type dopant. Subsequently, there were formed anMQW active layer 6 and an in-groove MQW layer 6a, both layers having acomposition of InGaAsP/InGaAs. These layers 6 and 6a were formed byfeeding predetermined amounts (flow rates) of TEGa, AsH₃, TMI, and PH₃in a manner such that 1.1 μm-InGaAsP thin film of 150 Å thick and InGaAsthin film of 20 Å thick were superimposed alternately in, for example, 4to 6 periods, to form a MQW striker. Further, a p-InP cladding layer 7,an n-InP buried layer 48, a p-InP layer 9, a p-InGaAsP contact layer 10were grown in this order to obtain a buried heterostructure as shown inFIG. 26D. Finally, a p-electrode 11 was formed on a part of the contactlayer 10 just above the ridge 3, and an n-electrode 12 was formed on theside of the semiconductor substrate. Thus, a light emitting diode chipof the present invention was fabricated (FIG. 26E).

The process according to the instant embodiment enables one to controlthe oscillating wavelength of a fabricated light emitting diode towithin the range of from 1.3 μm to 1.6 μm depending on the shape factorsof the semiconductor substrate, i.e., ridge width, dw, gap distance, dg,and ridge height, h.

FIG. 27 is a graph which illustrates the oscillating characteristics ofthe semiconductor light emitting diode fabricated by the above-describedprocess. More particularly, FIG. 27 illustrates the dependence ofoscillating efficiency (thick solid line) and wavelength shift (thinsolid line for anti-mesa type, and broken line for normal-mesa type) onthe thickness of a buffer layer. Oscillating efficiency was independentof the shape of the mesa and varied considerably depending on thethickness of the buffer layer in the same fashion regardless of whetherit was of a normal-mesa structure or of an anti-mesa structure, asindicated by thick solid line. From FIG. 27, it can be seen thatprovision of a buffer layer improved the oscillating characteristics byabout 5 times as high as that exhibited by the structure having nobuffer layer. This improvement is believed to be attributable to thephenomenon that the more improved quality of the crystal, the thickerthe buffer layer.

Further, it can be seen that assuming wavelength shift is a wavelengthregion in which the oscillating wavelength can be controlled, thewavelength shift decreased steeply with increase in the thickness of thebuffer layer in the case of a normal-mesa structure while this decreasewas relatively moderate in the case of an anti-mesa structure. Thistendency is believed to be attributable to the fact that the effectivedepth of groove (ridge height), h, decreases during the growth of thebuffer layer, and this decrease in effective ridge height is alleviatedmore in anti-mesa structures than in normal-mesa structures since theformer have greater volume than the latter. The oscillatingcharacteristics illustrated in FIG. 27 indicates that provision of abuffer layer and anti-mesa structure of a ridge structure are effectivefor further improvement of the oscillating characteristics of a lightemitting device.

Embodiment 13 Semiconductor Light Emitting Device Having a DamageAbsorbing Layer

It is known that while a nonplanar semiconductor substrate is beingformed, particularly when an oxide layer is formed and when dry etching,for example RIE with chlorine, damages tend to occur in thesemiconductor substrate and the quality of crystals grown thereondeteriorates, resulting in deteriorated characteristics of the resultinglight emitting device. The instant embodiment is intended to preventthis deterioration by provision of a damage absorbing layer on thesemiconductor substrate.

FIGS. 28A to 28D illustrate a fabrication process for fabricating asemiconductor light emitting diode having a damage absorbing layer.

First, a damage absorbing layer 20 of, for example, 1.3 μm-InGaAsP wasgrown to a thickness of 0.1 μm to 0.5 μm on a planar semiconductorsubstrate by MOVPE, and then a pattern of an oxide layer 2 was formed asshown in FIG. 28A. The substrate was dry etched to process the damageabsorbing layer 20 and the semiconductor substrate 1, followed byremoval of the oxide layer 2 to obtain a nonplanar semiconductorsubstrate with the damage absorbing layer 20 thereon as shown in FIG.28B. The substrate was selective etched with a sulfuric acid basedsolution to remove the damage absorbing layer 20. Thereafter, the sameprocedures as in Embodiment 11 or 12 were repeated to grow the samelayers 6, 7, 8, 5, 9, 10 as shown in FIG. 28C and provide the electrodes11 and 12 to fabricate a semiconductor emitting device as shown in FIG.28D. Here, the patterning and etching were performed so that the shapeof the grooves, particularly effective depth (ridge height), was notaffected by the provision of the damage absorbing layer as described inEmbodiment 12 above.

It was confirmed that the provision a damage absorbing layer not onlyimproved the oscillating efficiency of a semiconductor light emittingdevice but also increased its output due to improvement in quality ofthe crystal like Embodiments 11 and 12 above.

As described concretely in Embodiments 11 through 13, there can berealized, according to the present invention, a highly efficient lightemitting device whose oscillating wavelength can be controlled readily,and the use of a highly integrated optical functional device featured bya wide-band oscillating wavelength characteristics enables one torealize rapid progress in the development of measurement method,evaluation method, etc.

Embodiment 14 Integrated Optical Circuit

FIG. 29 is a schematic plan view showing an example of an integratedoptical circuit according to the present invention. In the instantembodiment, the integrated optical circuit were designed to use twowavelengths: 1.3 μm and 1.5 μm. Explanation will be made hereinaftertaking an example of a ping-pong two-way communication system usinglight having a wavelength in a 1.3 μm band, or broadcasting such as CATVusing light having a wavelength in a 1.5 μm band. Herein, by the terms"1.3 μm PD" and "1.5 μm PD", are meant PD for a 1.3 μm band light, andPD for 1.5 μm band light, respectively. Similarly, by the terms "1.3 μmLD" and "1.5 μm LD", are meant LD for a 1.3 μm band light, and LD for1.5 μm band light, respectively.

In FIG. 29, reference numeral 41 denotes a Y-branched optical waveguide,41a, 41b and 41c are portions of the waveguide 41, 42 is a 1.3 μm banddistributed feedback (DFB) semiconductor laser (LD), 43 is a monitor PD,44 is a 1.3 μm PD, 45 is a 1.3 μm residual light absorbing region orlayer, 46 is a 1.5 μm PD, 47 is a semiconductor substrate, 48 is a LDbranch portion, and 49 is a PD branch portion.

First explanation will be made of the operation of the integratedoptical circuit. In a 1.3 μm band ping-pong two-way opticalcommunication, transmission and reception are time-shared. They are notperformed simultaneously. In a 1.5 μm band, communication is of abroadcasting type, and only reception is performed. For transmission ofa 1.3 μm band light, laser beam generated in the DFB laser (1.3 μm LD)is propagated through the Y-branched waveguide 41 and outputted out ofthe integrated optical circuit. The power of the outputted light ismonitored by the monitor PD 43.

For reception of 1.3 μm and 1.5 μm light, a 1.3 μm light inputted in theintegrated optical circuit is propagated through a straight portion ofthe Y-branched waveguide 31 and split at the Y-branch and propagatedthrough two waveguides. The light to the LD branch 48 is inputted to theLD 42. However, no influence is given to the LD 42 since it is notactive while a 1.3 μm band light is being received. On the other hand,light to the PD branch 49 is absorbed by the 1.3 μm PD 44 and convertedinto photocurrent. Residual rays not absorbed are absorbed by theresidual light absorbing region 45 and, hence, no 1.3 μm light isinputted in the 1.5 μm PD 46. In other words, the 1.3 μm residual lightabsorbing region 45 serves as a wavelength filter which cuts 1.3 μmlight but allows 1.5 μm light to pass through it. When 1.5 μm light isinputted, it is propagated through a straight portion of the Y-branchedwaveguide 41 and split to two at the Y-branch, and then propagated intwo waveguides. Light to the LD branch 48 is inputted in 1.3 μm LD 42.In this occasion, 1.5 μm band light is not absorbed by the active layerof the 1.3 μm LD, giving no influence to the 1.3 μm LD 42. Light to thePD branch 49 is not absorbed by the residual light absorbing layer 45having a 1.3 μm band composition and therefore this light transmitsthrough the 1.3 μm PD 42 and 1.3 μm residual light absorbing region 45and reaches the 1.5 μm PD 46 where it is absorbed to generatephotocurrent.

Next, explanation will be made of a fabrication process for fabricatingthe integrated optical circuit of the instant embodiment with referenceto FIGS. 30 through 36.

1. Crystal growth:

1-1) On an n-InP substrate were grown a waveguide layer 14 consisting of1.1 μm-composition InGaAsP of 0.3 μm thick and an InP layer 5 of 20 nmthick to obtain a structure shown in FIG. 30.

1-2) Diffractive grating 13 having a pitch of 200 nm was formed in aregion expected to become a 1.3 μm LD (cf. 42 in FIG. 29) as shown inFIG. 31.

1-3) In a region of a flat surface 21 where a 1.5 μm PD (46 in FIG. 29)was to be formed, there was formed a ridge 3a which had a shape definedby a gap distance dg=2 μm, a ridge width dw=2 μm, and a ridge height h=2μm. On the other hand, in each of regions where 1.3 μm LD and monitorPD, and 1.3 μm PD were to be fabricated, respectively (42, 43 and 44 inFIG. 29), there was formed a ridge 3b which had a shape defined by a gapdistance dg₂ =10 μm, a ridge width dw=2 μm, and a ridge height h=2 μm.Thus, a structure as shown in FIG. 32 was obtained.

1-4) On the resulting nonplanar substrate, there was grown a MQWstructure 6 which was able to effectively give a bandgap of 1.25 μm onthe flat surface 21. As described above, the oscillating wavelengthshifted toward longer wavelength side on the ridges 3a and 3b. Thereappeared a 1.5 μm bandgap MQW structure 6a on the ridge 3a (dg₁ =2 μm)while a 1.3 μm bandgap MQW structure 6b was obtained on the ridge 3b(dg₂ =10 μm).

1-5) Embedding regrowth was performed to embed 1.5 μm PD, 1.3 μm LD and1.3 μm PD portions. This was done by MOVPE, preferably according to themethod described in Japanese Patent Application Laying-open No.Hei-5-102607 (1993). More particularly, a Zn-doped p-InP currentblocking layer 7, an Se-doped n-InP current confining layer 8 were grownin this order by MOVPE as shown in FIG. 34. The p-InP layer 7 and then-InP layer 8 served as a current constricting and light confininglayer. When the Se was doped in the n-InP layer 8 in an amount of notsmaller 5×10¹⁸ cm³¹ 3, the growth of the n-InP buried layer (currentconfining layer) 8 was suppressed on the ridge and, hence, a multilayerstructure was obtained in which only the p-InP current confining layergrew on the ridges 6a and 6b. Subsequently, a p-InP overcladding layer9, and a p-InGaAsP layer 10 were grown continuously by MOVPE.

1-6) The doped InP and 1.3 μm composition InGaAsP which were depositedon the Y-branched waveguides 41a, 41b and 41c at the time of embeddingregrowth were removed together with p-InGaAsP layer to form removedregions 22a, 22b and 22c as shown in FIG. 35.

1-7) As shown in FIG. 36, an undoped InP layer 23 (1,000 Å), an InGaAsPetch-stop layer 24 (200 Å) for selective etching, and an undoped InPlayer 25 were grown in this order in the removed regions 22a, 22b, and22c, respectively. This completed the crystal growth.

2. Electrode Formation:

2-1) p-Electrodes were formed on 1.3 μm LD 42, 1.3 μm PD 43, 44 and 45,and 1.5 μm PD 46, respectively, with AuZnNi/Au.

2-2) Waveguide portion 41 was wet etched in the form of stripes with aselective etching solution until the etch-stop layer was reached to forma ridge waveguide.

2-3) An insulator was deposited everywhere except for electrode portion.

2-4) Electrode pads were formed for attaching wiring.

2-5) The substrate was ground and an n-electrode was formed withAuGeNi/Au on the side of the substrate. This completed fabrication ofthe device.

Next, explanation will be made of the characteristics of the resultingdevice. FIG. 37 illustrates current vs. optical output characteristicswhen a 1.3 μm LD was oscillated. Threshold was 15 mA, and an output of 4mW was obtained at a current of 30 mA. Side mode suppression ratio was28 dB.

Then, explanation will be made of the characteristics of PD. Reversebias voltage of 1 V was applied to the 1.3 μm PD 44, and the 1.5 μm PD46, and the 1.3 μm residual light absorbing region 45 was earthed sothat photocurrent which was generated by residual rays would not flowinto other PD electrodes. In this state, light of 1.3 μm and light of1.5 μm were inputted from the left end face of the waveguide of thedevice and on the side of the 1.3 μm PD 44. On this occasion, 1.3 μm PD44 and 1.5 μm PD 46 absorbed light of 1.3 μm and 1.5 μm in wavelengthand photocurrent flowed in the both devices. Light of 1.3 μm, which wasnot absorbed by the 1.3 μm PD 44, was absorbed by the 1.3 μm residuallight absorbing region 45 and converted to photocurrent, which flew toground. As a result, almost no 1.3 μm light was inputted into the 1.5 μmPD 46. FIGS. 38A and 38B each illustrate dependence of photocurrent andcrosstalk for each PD on wavelength. FIG. 38A illustrates dependence ofcrosstalk of 1.3 μm PD while FIG. 38B illustrates dependence ofcrosstalk of 1.5 μm PD on wavelength. In measurement, crosstalk c wasdefined as follows. For 1.3 μm band: ##EQU1## For 1.5 μm band: ##EQU2##

FIGS. 38A and 38B reveals that good crosstalk characteristics of -24 dBwere observed in the both bands.

When 1.3 μm light and 1.5 μm light were inputted, light propagated alsoin the LD branch. However, influence to the LD was negligible even when1.3 μm light was being received since LD did not have to be operated.Further, as described above, 1.5 μm light gave no influence to LD thecharacteristics of the LD did not change while 1.5 μm light was beingreceived.

While a DFB laser was used in the instant embodiment, a DBR laser inwhich diffractive gratings are arranged on both sides of the activelayer may also be used.

In the instant embodiment, at first a 1.1 μm composition waveguide wasgrown on the substrate and used as a common waveguide. However, thewaveguide 41 (including waveguide portions 41a, 41b and 41c) may begrown simultaneously together with 1.3 μm PD's 44 and 45 and 1.5 μm PD46 since the composition of a crystal on the ridge can be varied from1.1 μm composition to 1.5 μm composition by optimizing the structure ofthe MQW. More particularly, as shown in FIGS. 39A and 39B, an MQWstructure can be grown on a ridge which has different ridge portionssandwiched by a groove 4a corresponding to the 1.5 μm PD 46, a groove 4bcorresponding to the 1.3 μm PD 44 and the 1.3 μm residual lightabsorbing region 45 and a grove 4c corresponding to the 1.1 μm waveguideportions 41a, 41b and 41c. For the LD branch, the Y branched waveguide41 may be grown simultaneously together with the 1.3 μm LD 42 and themonitor PD 43.

Embodiment 15

Referring to FIG. 40, explanation will be made of an integrated opticalcircuit according to the present invention.

In the arrangement shown in FIGS. 39A and 39B, there were usedsubstantially the same component devices as those used in Embodiment 14but arranged in a manner different from that in Embodiment 14. In FIG.40, reference numeral 51 is an active layer for LD, which active layerhad 1.3 μm effective bandgap, 52 is a diffractive grating forconstituting a DBR laser, 53 is a DBR laser portion, 54 is a curvedwaveguide, 55 is a 1.3 μm residual light absorbing region, 56 is a 1.5μm PD portion, 57 is a groove for cutting scattered light, 58 is asemiconductor substrate.

Explanation will be made of the operation of the integrated opticalcircuit of the instant embodiment. Transmission of light in a 1.3 μmband was performed using a DBR laser 53. The resonator of the laserdiode comprised a cleaved facet of the semiconductor substrate and adiffractive grating 52. Because of high reflectivity of the diffractive,laser beam was outputted almost exclusively from a left hand side faceof the device, but no output light on the side of the PD.

When the laser diode 53 was excited, spontaneous emission occurs as wellas lasing. The spontaneous emission light is not a guided light and,hence, it is possible to prevent it from being inputted into the 1.5 μmPD portion 56 by the introduction of a curved waveguide 54 (FIG. 40).While light propagates in a waveguide, there occurs scattered light.Inputting of such scattered light into a 1.5 μm PD portion 56 can beprevented by the provision of a scattered light cutoff groove 57 forcutting scattered light off. With the above-described construction,there can be reduced crosstalk to the 1.5 μm PD portion 56 when a 1.3 μmlight is transmitted.

On the other hand, upon reception of a 1.3 μm light, the laser diode 53serves as a detector. Optical system which uses the device of theinstant embodiment is a ping-pong two-way communication as describedabove, this type of application is possible. 1.3 μm light which has notbeen absorbed by the laser-and-detector propagates through the curvedwaveguide to reach the 1.3 μm residual light absorbing region 53 andabsorbed thereby, but does not enter the 1.5 μm PD portion 56. Uponreception of 1.5 μm light, the light propagates through the DBR laser(LD) portion 53, the curved waveguide 54 and 1.3 μm residual lightabsorbing region 55 to reach 1.5 μm PD portion 56 where the light isabsorbed and converted into photocurrent.

The integrated optical circuit according to the instant embodiment canbe fabricated substantially in the same manner as in Embodiment 14above. Also, the 1.3 μm LD and 1.3 μm PD as well as 1.5 μm PD had thesame characteristics as those used in Embodiment 14 above.

As described concretely with reference to Embodiments 14 and 15 above,an integrated optical circuit of the present invention which includes asemiconductor light emitting diode, a first semiconductor detector, anda second semiconductor detector which detects light having a wavelengthlonger than that of light which can be detected by the first detector,and a wavelength filter provided between the first and the seconddetectors, and optical waveguides connecting these components to eachother can be operated as a transmission device for transmitting light offirst wavelength and as a detector for detecting light of secondwavelength.

Embodiment 16 Spot Size Converter

FIGS. 41 through 43 show a spot size converter according to oneembodiment of the present invention. The spot size converter of theinstant embodiment was fabricated as follows. First, there was formed onan InP substrate 1 a 2 μm thick 5 μm composition InGaAsP layer 61. Apart of the InGaAsP layer 61 was removed by RIE etching with chlorinegas so that two tapered grooves 4d and a ridge 3d were formed. Thus,there was obtained a structure as shown in FIG. 41. On the front face ofthe substrate 1, the width of the ridge 3d was 5 μm, and the width (gapdistance) of the groove d was 10 μm. On the rear face of the substrate,the width of the ridge 3d was 1.5 μm and the gap distance of the groove4d was 3 μm. The ridge had a uniform height overall the substrate, whichwas 2 μm. Near the rear face of the substrate 1, there was formed aportion where the ridge 3d and the grooves 4d are parallel to eachother. However, formation of this parallel portion is not mandatory. Onthe contrary, such a parallel portion may be provided near the frontface of the substrate 1.

Next, the above-described structure having the substrate 1 and the ridge3d formed thereon was used as a substrate for epitaxial growth. That is,a 150 Å thick InGaAsP layer of 1.1. μm composition was grown on thestructure by MOVPE or MOMBE to obtain a barrier layer 3d. Then, on thebarrier layer 6d, there was grown a 17 Å thick InGaAs well layer 6e.This procedure was repeated until there was obtained a quantum wellstructure 6 of a total thickness of about 0.3 μm. Subsequently, a 2 μmthick InGaAsP layer 62 of 1.35 μm composition was grown, and finally anInP layer 63 was grown to a thickness of 2 μm. FIGS. 42 and 43 showfront end face and rear end face, respectively, of the spot sizeconverter, with schematic enlarged views of the resulting MQW structureat its frond end face and rear end face, respectively.

The width of the ridge 3d on the rear end face of the device was 1.5 μmas described above, and the width of the grooves 4d was 3 μm. From this,it follows that the bandgap of the MQW structure 6 became 1.0 μm as willbe apparent from FIG. 7 illustrating dependence of photoluminescencepeak wavelength on the ridge shape factors. Further, since the MQW layer6 on the side surface of the ridge 3d was thinner than on the uppersurface of the ridge 3d as shown in FIG. 43, the guided light wasconfined in the MQW layer 64 on the ridge as a core, or a cavity, havinga thickness of about 0.3 μm and a width of about 1.5 μm. The spot sizewas within the range of about 1.5 μm to about 2 μm, which correspondedto the spot size of the semiconductor device.

Similarly, the width of the ridge 3d on the front end face of the devicewas 5 μm as described above while the width of the grooves was 10 μm.Hence, the bandgap of the MQW structure 6 was 1.35 μm on the front endface of the device as will be apparent from the dependence ofphotoluminescence peak wavelength on the ridge shape factors asillustrated in FIG. 7. As a result, the refractive index of the MQWstructure 6 became substantially the same as or smaller than the layer61 constituting the ridge 3d. Therefore, on the front end face of thedevice, there was formed a buried ridge structure of about 4.5 μm thickand about 5 μm wide including the layers 61, 6 and 62 as a core and thelayers 1 and 63 as upper and lower cladding layers, respectively. Inthis case, the center of the core layer was in the layer 6, resulting inthat guided light was able to be transmitted from a state in which it isconfined by the MQW structure 64 on the rear end face of the device toanother state in which it is confined by the layers 61, 6 and 62 on thefront end face of the device without causing any misalignment. As aresult, on the front end face of the device, there was formed awaveguide having a spot size enlarged to 4 to 5 μm, which was of asimilar dimension as optical fibers.

A flat end fiber was connected to the front end face of the resultingdevice and connection loss was measured. As a result, it was confirmedthat the loss was not higher than 0.5 dB, and allowance of connectionwas +2.4 μm for a tolerance of 1 dB.

In the instant embodiment, wile the width of the ridge was enlarged to 5μm on the front end face of the device, the effect of spot sizeenlargement can be obtained even when the ridge width was kept at aconstant value if the groove width (gap distance) was varied asconfirmed from the results illustrated in FIG. 7. This is believed to beattributable to the fact that difference in refractive index between thecore and cladding is small in a direction horizontal to the surface ofthe substrate on the front end face of the device and, hence, guidedlight evanesces to the region of cladding so that the effective spotsize can be enlarged.

Embodiment 17

FIGS. 44 through 46 illustrate a spot size converter according toanother embodiment of the present invention. As a substrate for growinga MQW layer, a nonplanar structure similar to that used in Embodiment 16was used. In the instant embodiment, a diffractive grating 13 was formedon a portion of the upper surface of the ridge 3d, as shown in FIG. 44.The layers 1 and 61 were doped with impurities so that they wereconverted to n-type in conductivity. After a MQW layer 6 was grown inthe same manner as in Embodiment 16, a p-InGaAsP layer 65 was grown asproposed by Kondo, et al. (Japanese Patent Application Laying-open No.Hei-5-102607 (1991)). That is, after the MQW layer 6 was grown, a stripemask (not shown) was formed only above a MQW layer 64 that waspositioned on the ridge 3d. the MQW layer 6 around the layer 64 wasetched off using a RIE apparatus to render the layer 64 mesa-structure.Thereafter, the mask was removed.

Subsequently, on the thus processed surface, an n-InP or InGaAsP layer66 was grown, followed by growing a p-InGaAsP layer 62 and an InP layer63 as well as by forming an electrode 67 on a part of the uppermostlayer. As a result, the structure as shown in FIG. 44 was formed on therear end face of the device. This was a laser diode structure includingthe n-InP layers 1 and 63 as current blocking layers, which operated asa DFB laser diode when electrodes were formed on both upper and lowerfaces of the device.

On the other hand, the refractive index distribution on the front endface of the device was substantially the same as that of the deviceobtained in Embodiment 16 and, hence, the device of the instantembodiment also had a waveguide structure with an enlarged spot size.

Therefore, in the instant embodiment, monolithically integrated opticaldevice having a spot size-enlarged waveguide and a DFB laser can befabricated by a single epitaxial growth.

In the instant embodiment, there may be fabricated a device using asubstrate in which the thickness of the layer 61 is 4 μm of which 2 μmhas been processed to form a ridge, as well as p-InP layers as thelayers 62 and 65 and an n-InP layer as the layer 66. In this case, thecenters of the spots of the guided light does not align on the front andrear end faces. However, this structure is advantageous in that thestructure of the p-cladding layer is simplified and confinement ofcurrent by pnpn structure can be performed effectively.

In the instant embodiment, when no diffractive grating 13 is fabricated,the device can be used as is as a photodetector diode so that there canbe realized a waveguide type photodiode having high coupling efficiencywith optical fibers.

Embodiment 18

FIGS. 47 through 49 illustrate a spot size converter according to stillanother embodiment of the present invention. The device of the instantembodiment has a structure similar to that shown in FIG. 44 through 46except that in the instant embodiment, a ridge shape (ridge width andgap distance)-modulated structure was provided also on the rear end faceof the device in addition to that on the front end face of the device,with the former being arranged in a direction opposite to the former(FIG. 47). FIG. 48 shows structures of the device on its front and rearend faces after epitaxial growth performed in the same manner as in thepreceding embodiments. FIG. 49 is a cross sectional view showing acentral region of the device where the ridge width and gap distance arerelatively small. In FIG. 49, same or like structures are indicated bysame references, and detailed description are omitted here.

Anti-reflection coating was provided on both end faces of the devicehaving the above-described construction and p- and n-electrodes (notshown) were provide on both faces of the substrate. Upon injection ofcurrent to the electrode, a semiconductor amplifier having high couplingefficiency with optical fibers can be obtained.

Further, in the instant embodiment, when polarity of applied voltage isreversed, an optical modulator utilizing confined Stark effect can berealized.

In the instant embodiment, while the propagation of 1.5 μm light wasexplained taking an example of InGaAsP based materials, there may beused also other materials, for example, InGaAlAs based materials, orboth of the InGaAsP based materials and InAlGaAs based materials.Further, in other wavelength regions such as 1.3 μm, similar effects canbe obtained by selecting types of material and composition.

As described concretely in Embodiments 16 through 18, the present invention enables enlargement of spot size of the waveguide in an opticaldevice by a simple fabrication process. As a result, there can berealized integration of semiconductor devices with high performance andhigh reliability.

The present invention has been described in detail with respect to anembodiment, and it will now be apparent from-the foregoing to thoseskilled in the art that changes and modifications may be made withoutdeparting from the invention in its broader aspects, and it is theintention, therefore, in the appended claims to cover all such changesand modifications as fall within the true spirit of the invention.

What is claimed is:
 1. A method for fabricating an optical device,comprising the steps of:providing a nonplanar semiconductor substratehaving a ridge of which a ridge width is from 1 μm to 10 μm, a ridgeheight is from 1 μm to 5 μm, and a gap distance is from 1 μm to 10 μm;and growing a strained multi-quantum well layer on said nonplanarsemiconductor substrate by metalorganic vapor phase epitaxy to form anfunctional optical layer.
 2. The method for fabricating an opticaldevice as claimed in claim 1, wherein the step of providing saidnonplanar semiconductor substrate includes the steps of:providing aplanar semiconductor substrate; forming a semiconductor protective thinfilm layer on said planar semiconductor substrate having a compositiondifferent from that of said planar semiconductor substrate; andprocessing said planar semiconductor substrate and said protective thinfilm layer to render said planar semiconductor substrate nonplanar.
 3. Amethod for controlling optical characteristics of an optical device, themethod comprising the steps of:providing a nonplanar semiconductorsubstrate having a ridge of a selected shape of which a ridge width isfrom 1 μm to 10 μm, a ridge height is from 1 μm to 5 μm, and a gapdistance is from 1 μm to 10 μm; growing a multi-quantum well structureon said nonplanar semiconductor substrate by metalorganic vapor phaseepitaxy to form a functional optical layer; and varying a composition ofsaid multiquantum well structure formed on said ridge to change opticalcharacteristics thereof.
 4. The method for controlling opticalcharacteristics of an optical device as claimed in claim 3, wherein saidfunctional optical layer is an optical waveguide layer, and wherein:saidcomposition varying step includes varying at least one of said ridgewidth, ridge height and gap distance from one end toward another end ofsaid ridge on which said optical waveguide layer is formed to vary oneof bandgap and refractive index characteristics of said opticalwaveguide layer.
 5. The method for controlling optical characteristicsof an optical device as claimed in claim 3,wherein said opticalcharacteristics comprise one of light emitting characteristics anddetecting characteristics of said optical device; and wherein at leastone of said ridge width, ridge height and gap distance is varied in alongitudinal direction of said ridge, so that said light emittingcharacteristics and said detecting characteristics of said opticaldevice can be varied.
 6. The method for controlling opticalcharacteristics of an optical device as claimed in claim 3, wherein saidproviding step includes the step of providing a plurality of said ridgesin parallel spaced relation, andwherein said optical characteristicscomprise one of light emitting characteristics and light detectingcharacteristics of said optical device; and wherein at least one of saidridge width, ridge height and gap distance is varied in a transversedirection to form an array-like optical functional device, so that oneof light emitting characteristics and light detecting characteristics ofsaid optical device can be varied.